File posix-memalign-fragmentation.patch of Package glibc

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File posix-memalign-fragmentation.patch of Package glibc (Revision a698deb5592e31045a2f072fc47d17fc)

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From 542b1105852568c3ebc712225ae78b8c8ba31a78 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Fri, 11 Aug 2023 11:18:17 +0200
Subject: [PATCH] malloc: Enable merging of remainders in memalign (bug 30723)

Previously, calling _int_free from _int_memalign could put remainders
into the tcache or into fastbins, where they are invisible to the
low-level allocator.  This results in missed merge opportunities
because once these freed chunks become available to the low-level
allocator, further memalign allocations (even of the same size are)
likely obstructing merges.

Furthermore, during forwards merging in _int_memalign, do not
completely give up when the remainder is too small to serve as a
chunk on its own.  We can still give it back if it can be merged
with the following unused chunk.  This makes it more likely that
memalign calls in a loop achieve a compact memory layout,
independently of initial heap layout.

Drop some useless (unsigned long) casts along the way, and tweak
the style to more closely match GNU on changed lines.

Reviewed-by: DJ Delorie <dj@redhat.com>
---
 malloc/malloc.c | 197 +++++++++++++++++++++++++++++-------------------
 1 file changed, 121 insertions(+), 76 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index e2f1a615a4..948f9759af 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1086,6 +1086,11 @@ typedef struct malloc_chunk* mchunkptr;
 
 static void*  _int_malloc(mstate, size_t);
 static void     _int_free(mstate, mchunkptr, int);
+static void _int_free_merge_chunk (mstate, mchunkptr, INTERNAL_SIZE_T);
+static INTERNAL_SIZE_T _int_free_create_chunk (mstate,
+					       mchunkptr, INTERNAL_SIZE_T,
+					       mchunkptr, INTERNAL_SIZE_T);
+static void _int_free_maybe_consolidate (mstate, INTERNAL_SIZE_T);
 static void*  _int_realloc(mstate, mchunkptr, INTERNAL_SIZE_T,
 			   INTERNAL_SIZE_T);
 static void*  _int_memalign(mstate, size_t, size_t);
@@ -4637,31 +4642,52 @@ _int_free (mstate av, mchunkptr p, int have_lock)
     if (!have_lock)
       __libc_lock_lock (av->mutex);
 
-    nextchunk = chunk_at_offset(p, size);
-
-    /* Lightweight tests: check whether the block is already the
-       top block.  */
-    if (__glibc_unlikely (p == av->top))
-      malloc_printerr ("double free or corruption (top)");
-    /* Or whether the next chunk is beyond the boundaries of the arena.  */
-    if (__builtin_expect (contiguous (av)
-			  && (char *) nextchunk
-			  >= ((char *) av->top + chunksize(av->top)), 0))
-	malloc_printerr ("double free or corruption (out)");
-    /* Or whether the block is actually not marked used.  */
-    if (__glibc_unlikely (!prev_inuse(nextchunk)))
-      malloc_printerr ("double free or corruption (!prev)");
-
-    nextsize = chunksize(nextchunk);
-    if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
-	|| __builtin_expect (nextsize >= av->system_mem, 0))
-      malloc_printerr ("free(): invalid next size (normal)");
+    _int_free_merge_chunk (av, p, size);
 
-    free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
+    if (!have_lock)
+      __libc_lock_unlock (av->mutex);
+  }
+  /*
+    If the chunk was allocated via mmap, release via munmap().
+  */
+
+  else {
+    munmap_chunk (p);
+  }
+}
+
+/* Try to merge chunk P of SIZE bytes with its neighbors.  Put the
+   resulting chunk on the appropriate bin list.  P must not be on a
+   bin list yet, and it can be in use.  */
+static void
+_int_free_merge_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size)
+{
+  mchunkptr nextchunk = chunk_at_offset(p, size);
+
+  /* Lightweight tests: check whether the block is already the
+     top block.  */
+  if (__glibc_unlikely (p == av->top))
+    malloc_printerr ("double free or corruption (top)");
+  /* Or whether the next chunk is beyond the boundaries of the arena.  */
+  if (__builtin_expect (contiguous (av)
+			&& (char *) nextchunk
+			>= ((char *) av->top + chunksize(av->top)), 0))
+    malloc_printerr ("double free or corruption (out)");
+  /* Or whether the block is actually not marked used.  */
+  if (__glibc_unlikely (!prev_inuse(nextchunk)))
+    malloc_printerr ("double free or corruption (!prev)");
+
+  INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
+  if (__builtin_expect (chunksize_nomask (nextchunk) <= CHUNK_HDR_SZ, 0)
+      || __builtin_expect (nextsize >= av->system_mem, 0))
+    malloc_printerr ("free(): invalid next size (normal)");
+
+  free_perturb (chunk2mem(p), size - CHUNK_HDR_SZ);
 
-    /* consolidate backward */
-    if (!prev_inuse(p)) {
-      prevsize = prev_size (p);
+  /* Consolidate backward.  */
+  if (!prev_inuse(p))
+    {
+      INTERNAL_SIZE_T prevsize = prev_size (p);
       size += prevsize;
       p = chunk_at_offset(p, -((long) prevsize));
       if (__glibc_unlikely (chunksize(p) != prevsize))
@@ -4669,9 +4695,25 @@ _int_free (mstate av, mchunkptr p, int have_lock)
       unlink_chunk (av, p);
     }
 
-    if (nextchunk != av->top) {
+  /* Write the chunk header, maybe after merging with the following chunk.  */
+  size = _int_free_create_chunk (av, p, size, nextchunk, nextsize);
+  _int_free_maybe_consolidate (av, size);
+}
+
+/* Create a chunk at P of SIZE bytes, with SIZE potentially increased
+   to cover the immediately following chunk NEXTCHUNK of NEXTSIZE
+   bytes (if NEXTCHUNK is unused).  The chunk at P is not actually
+   read and does not have to be initialized.  After creation, it is
+   placed on the appropriate bin list.  The function returns the size
+   of the new chunk.  */
+static INTERNAL_SIZE_T
+_int_free_create_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size,
+			mchunkptr nextchunk, INTERNAL_SIZE_T nextsize)
+{
+  if (nextchunk != av->top)
+    {
       /* get and clear inuse bit */
-      nextinuse = inuse_bit_at_offset(nextchunk, nextsize);
+      bool nextinuse = inuse_bit_at_offset (nextchunk, nextsize);
 
       /* consolidate forward */
       if (!nextinuse) {
@@ -4686,8 +4728,8 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 	been given one chance to be used in malloc.
       */
 
-      bck = unsorted_chunks(av);
-      fwd = bck->fd;
+      mchunkptr bck = unsorted_chunks (av);
+      mchunkptr fwd = bck->fd;
       if (__glibc_unlikely (fwd->bk != bck))
 	malloc_printerr ("free(): corrupted unsorted chunks");
       p->fd = fwd;
@@ -4706,61 +4748,52 @@ _int_free (mstate av, mchunkptr p, int have_lock)
       check_free_chunk(av, p);
     }
 
-    /*
-      If the chunk borders the current high end of memory,
-      consolidate into top
-    */
-
-    else {
+  else
+    {
+      /* If the chunk borders the current high end of memory,
+	 consolidate into top.  */
       size += nextsize;
       set_head(p, size | PREV_INUSE);
       av->top = p;
       check_chunk(av, p);
     }
 
-    /*
-      If freeing a large space, consolidate possibly-surrounding
-      chunks. Then, if the total unused topmost memory exceeds trim
-      threshold, ask malloc_trim to reduce top.
-
-      Unless max_fast is 0, we don't know if there are fastbins
-      bordering top, so we cannot tell for sure whether threshold
-      has been reached unless fastbins are consolidated.  But we
-      don't want to consolidate on each free.  As a compromise,
-      consolidation is performed if FASTBIN_CONSOLIDATION_THRESHOLD
-      is reached.
-    */
+  return size;
+}
 
-    if ((unsigned long)(size) >= FASTBIN_CONSOLIDATION_THRESHOLD) {
+/* If freeing a large space, consolidate possibly-surrounding
+   chunks.  Then, if the total unused topmost memory exceeds trim
+   threshold, ask malloc_trim to reduce top.  */
+static void
+_int_free_maybe_consolidate (mstate av, INTERNAL_SIZE_T size)
+{
+  /* Unless max_fast is 0, we don't know if there are fastbins
+     bordering top, so we cannot tell for sure whether threshold has
+     been reached unless fastbins are consolidated.  But we don't want
+     to consolidate on each free.  As a compromise, consolidation is
+     performed if FASTBIN_CONSOLIDATION_THRESHOLD is reached.  */
+  if (size >= FASTBIN_CONSOLIDATION_THRESHOLD)
+    {
       if (atomic_load_relaxed (&av->have_fastchunks))
 	malloc_consolidate(av);
 
-      if (av == &main_arena) {
+      if (av == &main_arena)
+	{
 #ifndef MORECORE_CANNOT_TRIM
-	if ((unsigned long)(chunksize(av->top)) >=
-	    (unsigned long)(mp_.trim_threshold))
-	  systrim(mp_.top_pad, av);
+	  if (chunksize (av->top) >= mp_.trim_threshold)
+	    systrim (mp_.top_pad, av);
 #endif
-      } else {
-	/* Always try heap_trim(), even if the top chunk is not
-	   large, because the corresponding heap might go away.  */
-	heap_info *heap = heap_for_ptr(top(av));
+	}
+      else
+	{
+	  /* Always try heap_trim, even if the top chunk is not large,
+	     because the corresponding heap might go away.  */
+	  heap_info *heap = heap_for_ptr (top (av));
 
-	assert(heap->ar_ptr == av);
-	heap_trim(heap, mp_.top_pad);
-      }
+	  assert (heap->ar_ptr == av);
+	  heap_trim (heap, mp_.top_pad);
+	}
     }
-
-    if (!have_lock)
-      __libc_lock_unlock (av->mutex);
-  }
-  /*
-    If the chunk was allocated via mmap, release via munmap().
-  */
-
-  else {
-    munmap_chunk (p);
-  }
 }
 
 /*
@@ -5221,7 +5254,7 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
                 (av != &main_arena ? NON_MAIN_ARENA : 0));
       set_inuse_bit_at_offset (newp, newsize);
       set_head_size (p, leadsize | (av != &main_arena ? NON_MAIN_ARENA : 0));
-      _int_free (av, p, 1);
+      _int_free_merge_chunk (av, p, leadsize);
       p = newp;
 
       assert (newsize >= nb &&
@@ -5232,15 +5265,27 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
   if (!chunk_is_mmapped (p))
     {
       size = chunksize (p);
-      if ((unsigned long) (size) > (unsigned long) (nb + MINSIZE))
+      mchunkptr nextchunk = chunk_at_offset(p, size);
+      INTERNAL_SIZE_T nextsize = chunksize(nextchunk);
+      if (size > nb)
         {
           remainder_size = size - nb;
-          remainder = chunk_at_offset (p, nb);
-          set_head (remainder, remainder_size | PREV_INUSE |
-                    (av != &main_arena ? NON_MAIN_ARENA : 0));
-          set_head_size (p, nb);
-          _int_free (av, remainder, 1);
-        }
+	  if (remainder_size >= MINSIZE
+	      || nextchunk == av->top
+	      || !inuse_bit_at_offset (nextchunk, nextsize))
+	    {
+	      /* We can only give back the tail if it is larger than
+		 MINSIZE, or if the following chunk is unused (top
+		 chunk or unused in-heap chunk).  Otherwise we would
+		 create a chunk that is smaller than MINSIZE.  */
+	      remainder = chunk_at_offset (p, nb);
+	      set_head_size (p, nb);
+	      remainder_size = _int_free_create_chunk (av, remainder,
+						       remainder_size,
+						       nextchunk, nextsize);
+	      _int_free_maybe_consolidate (av, remainder_size);
+	    }
+	}
     }
 
   check_inuse_chunk (av, p);
-- 
2.41.0

From 0dc7fc1cf094406a138e4d1bcf9553e59edcf89d Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Thu, 10 Aug 2023 19:36:56 +0200
Subject: [PATCH] malloc: Remove bin scanning from memalign (bug 30723)

On the test workload (mpv --cache=yes with VP9 video decoding), the
bin scanning has a very poor success rate (less than 2%).  The tcache
scanning has about 50% success rate, so keep that.

Update comments in malloc/tst-memalign-2 to indicate the purpose
of the tests.  Even with the scanning removed, the additional
merging opportunities since commit 542b1105852568c3ebc712225ae78b
("malloc: Enable merging of remainders in memalign (bug 30723)")
are sufficient to pass the existing large bins test.

Remove leftover variables from _int_free from refactoring in the
same commit.

Reviewed-by: DJ Delorie <dj@redhat.com>
---
 malloc/malloc.c         | 169 ++--------------------------------------
 malloc/tst-memalign-2.c |   7 +-
 2 files changed, 10 insertions(+), 166 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 948f9759af..d0bbbf3710 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -4488,12 +4488,6 @@ _int_free (mstate av, mchunkptr p, int have_lock)
 {
   INTERNAL_SIZE_T size;        /* its size */
   mfastbinptr *fb;             /* associated fastbin */
-  mchunkptr nextchunk;         /* next contiguous chunk */
-  INTERNAL_SIZE_T nextsize;    /* its size */
-  int nextinuse;               /* true if nextchunk is used */
-  INTERNAL_SIZE_T prevsize;    /* size of previous contiguous chunk */
-  mchunkptr bck;               /* misc temp for linking */
-  mchunkptr fwd;               /* misc temp for linking */
 
   size = chunksize (p);
 
@@ -5032,42 +5026,6 @@ _int_realloc (mstate av, mchunkptr oldp, INTERNAL_SIZE_T oldsize,
    ------------------------------ memalign ------------------------------
  */
 
-/* Returns 0 if the chunk is not and does not contain the requested
-   aligned sub-chunk, else returns the amount of "waste" from
-   trimming.  NB is the *chunk* byte size, not the user byte
-   size.  */
-static size_t
-chunk_ok_for_memalign (mchunkptr p, size_t alignment, size_t nb)
-{
-  void *m = chunk2mem (p);
-  INTERNAL_SIZE_T size = chunksize (p);
-  void *aligned_m = m;
-
-  if (__glibc_unlikely (misaligned_chunk (p)))
-    malloc_printerr ("_int_memalign(): unaligned chunk detected");
-
-  aligned_m = PTR_ALIGN_UP (m, alignment);
-
-  INTERNAL_SIZE_T front_extra = (intptr_t) aligned_m - (intptr_t) m;
-
-  /* We can't trim off the front as it's too small.  */
-  if (front_extra > 0 && front_extra < MINSIZE)
-    return 0;
-
-  /* If it's a perfect fit, it's an exception to the return value rule
-     (we would return zero waste, which looks like "not usable"), so
-     handle it here by returning a small non-zero value instead.  */
-  if (size == nb && front_extra == 0)
-    return 1;
-
-  /* If the block we need fits in the chunk, calculate total waste.  */
-  if (size > nb + front_extra)
-    return size - nb;
-
-  /* Can't use this chunk.  */
-  return 0;
-}
-
 /* BYTES is user requested bytes, not requested chunksize bytes.  */
 static void *
 _int_memalign (mstate av, size_t alignment, size_t bytes)
@@ -5082,7 +5040,6 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
   mchunkptr remainder;            /* spare room at end to split off */
   unsigned long remainder_size;   /* its size */
   INTERNAL_SIZE_T size;
-  mchunkptr victim;
 
   nb = checked_request2size (bytes);
   if (nb == 0)
@@ -5101,129 +5058,13 @@ _int_memalign (mstate av, size_t alignment, size_t bytes)
      we don't find anything in those bins, the common malloc code will
      scan starting at 2x.  */
 
-  /* This will be set if we found a candidate chunk.  */
-  victim = NULL;
-
-  /* Fast bins are singly-linked, hard to remove a chunk from the middle
-     and unlikely to meet our alignment requirements.  We have not done
-     any experimentation with searching for aligned fastbins.  */
-
-  if (av != NULL)
-    {
-      int first_bin_index;
-      int first_largebin_index;
-      int last_bin_index;
-
-      if (in_smallbin_range (nb))
-	first_bin_index = smallbin_index (nb);
-      else
-	first_bin_index = largebin_index (nb);
-
-      if (in_smallbin_range (nb * 2))
-	last_bin_index = smallbin_index (nb * 2);
-      else
-	last_bin_index = largebin_index (nb * 2);
-
-      first_largebin_index = largebin_index (MIN_LARGE_SIZE);
-
-      int victim_index;                 /* its bin index */
-
-      for (victim_index = first_bin_index;
-	   victim_index < last_bin_index;
-	   victim_index ++)
-	{
-	  victim = NULL;
-
-	  if (victim_index < first_largebin_index)
-	    {
-	      /* Check small bins.  Small bin chunks are doubly-linked despite
-		 being the same size.  */
-
-	      mchunkptr fwd;                    /* misc temp for linking */
-	      mchunkptr bck;                    /* misc temp for linking */
-
-	      bck = bin_at (av, victim_index);
-	      fwd = bck->fd;
-	      while (fwd != bck)
-		{
-		  if (chunk_ok_for_memalign (fwd, alignment, nb) > 0)
-		    {
-		      victim = fwd;
-
-		      /* Unlink it */
-		      victim->fd->bk = victim->bk;
-		      victim->bk->fd = victim->fd;
-		      break;
-		    }
-
-		  fwd = fwd->fd;
-		}
-	    }
-	  else
-	    {
-	      /* Check large bins.  */
-	      mchunkptr fwd;                    /* misc temp for linking */
-	      mchunkptr bck;                    /* misc temp for linking */
-	      mchunkptr best = NULL;
-	      size_t best_size = 0;
-
-	      bck = bin_at (av, victim_index);
-	      fwd = bck->fd;
+  /* Call malloc with worst case padding to hit alignment. */
+  m = (char *) (_int_malloc (av, nb + alignment + MINSIZE));
 
-	      while (fwd != bck)
-		{
-		  int extra;
-
-		  if (chunksize (fwd) < nb)
-		    break;
-		  extra = chunk_ok_for_memalign (fwd, alignment, nb);
-		  if (extra > 0
-		      && (extra <= best_size || best == NULL))
-		    {
-		      best = fwd;
-		      best_size = extra;
-		    }
+  if (m == 0)
+    return 0;           /* propagate failure */
 
-		  fwd = fwd->fd;
-		}
-	      victim = best;
-
-	      if (victim != NULL)
-		{
-		  unlink_chunk (av, victim);
-		  break;
-		}
-	    }
-
-	  if (victim != NULL)
-	    break;
-	}
-    }
-
-  /* Strategy: find a spot within that chunk that meets the alignment
-     request, and then possibly free the leading and trailing space.
-     This strategy is incredibly costly and can lead to external
-     fragmentation if header and footer chunks are unused.  */
-
-  if (victim != NULL)
-    {
-      p = victim;
-      m = chunk2mem (p);
-      set_inuse (p);
-      if (av != &main_arena)
-	set_non_main_arena (p);
-    }
-  else
-    {
-      /* Call malloc with worst case padding to hit alignment. */
-
-      m = (char *) (_int_malloc (av, nb + alignment + MINSIZE));
-
-      if (m == 0)
-	return 0;           /* propagate failure */
-
-      p = mem2chunk (m);
-    }
+  p = mem2chunk (m);
 
   if ((((unsigned long) (m)) % alignment) != 0)   /* misaligned */
     {
diff --git a/malloc/tst-memalign-2.c b/malloc/tst-memalign-2.c
index f229283dbf..ecd6fa249e 100644
--- a/malloc/tst-memalign-2.c
+++ b/malloc/tst-memalign-2.c
@@ -86,7 +86,8 @@ do_test (void)
       TEST_VERIFY (tcache_allocs[i].ptr1 == tcache_allocs[i].ptr2);
     }
 
-  /* Test for non-head tcache hits.  */
+  /* Test for non-head tcache hits.  This exercises the memalign
+     scanning code to find matching allocations.  */
   for (i = 0; i < array_length (ptr); ++ i)
     {
       if (i == 4)
@@ -113,7 +114,9 @@ do_test (void)
   free (p);
   TEST_VERIFY (count > 0);
 
-  /* Large bins test.  */
+  /* Large bins test.  This verifies that the over-allocated parts
+     that memalign releases for future allocations can be reused by
+     memalign itself at least in some cases.  */
 
   for (i = 0; i < LN; ++ i)
     {
-- 
2.41.0

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File posix-memalign-fragmentation.patch of Package glibc (Revision a698deb5592e31045a2f072fc47d17fc)

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