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One of the things on the current must-fix list is:
> - Overcommit accounting gets wrong answers
>
> - underestimates reclaimable slab, gives bogus failures when
> dcache&icache are large.
More comments from Andrew:
> If the cache slab fragmentation is bad it can be hugely wrong.
> A factor of ten has been observed (rarely). Factors of two happen
> often.
...
> > But, if prune_[di]cache() will only touch those which are being
> > counted by nr_unused, how can we be more aggressive?
>
> Well, just by assuming all slab is reclaimable is one way.
>
> The problem with that is that to read slab accounting we need to do
> get_page_state(), which is too expensive to be called for every mmap()
> on big SMP.
Instead of going through get_page_state(), the following code keeps
track of entries as their space is allocated in the slab via
{con,de}structors. It _will_ overestimate the amount of reclaimable
slab but, previously, using the .nr_unused stat, this number was
underestimated and caused too many good allocations to fail. This
assumes that every dentry/inode allocated in the slab is reclaimable,
which they probably will be if we get deperate enough anyway.
and, as for the counter type being an atomic_t:
> Andrew Morton wrote:
> > Dave Hansen wrote:
> > An atomic_t might be a good idea, but I'm a bit worried that 24 bits
> > might not be enough. At 160 bytes/dentry, that's 2.5GB of dentries
> > before the counter overflows. I would imagine that we'll run out of
> > plenty of other things before we get to _that_ many dentries.
>
> The 24-bit thing is only on sparc32. I don't think 2G of dentries
> is possible on sparc32 anyway.
The attached patch is against 2.5.69-mm7.
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diff -rup linux-2.5.69-mm8-clean/fs/dcache.c linux-2.5.69-mm8-dcache-count/fs/dcache.c --- linux-2.5.69-mm8-clean/fs/dcache.c Mon May 19 13:25:53 2003 +++ linux-2.5.69-mm8-dcache-count/fs/dcache.c Mon May 19 13:45:05 2003 @@ -1529,6 +1529,16 @@ out: return ino; } +void d_ctor(void * objp, struct kmem_cache_s *cachep, unsigned long dflags) +{ + atomic_inc(&dentry_stat.nr_alloced); +} + +void d_dtor(void * objp, struct kmem_cache_s *cachep, unsigned long dflags) +{ + atomic_dec(&dentry_stat.nr_alloced); +} + static void __init dcache_init(unsigned long mempages) { struct hlist_head *d; @@ -1548,7 +1558,7 @@ static void __init dcache_init(unsigned sizeof(struct dentry), 0, SLAB_HWCACHE_ALIGN, - NULL, NULL); + d_ctor, d_dtor); if (!dentry_cache) panic("Cannot create dentry cache"); diff -rup linux-2.5.69-mm8-clean/fs/inode.c linux-2.5.69-mm8-dcache-count/fs/inode.c --- linux-2.5.69-mm8-clean/fs/inode.c Mon May 19 13:25:54 2003 +++ linux-2.5.69-mm8-dcache-count/fs/inode.c Mon May 19 16:23:14 2003 @@ -197,6 +197,13 @@ static void init_once(void * foo, kmem_c if ((flags & (SLAB_CTOR_VERIFY|SLAB_CTOR_CONSTRUCTOR)) == SLAB_CTOR_CONSTRUCTOR) inode_init_once(inode); + + atomic_inc(&inodes_stat.nr_alloced); +} + +void inode_dtor(void * objp, struct kmem_cache_s *cachep, unsigned long dflags) +{ + atomic_dec(&inodes_stat.nr_alloced); } /* diff -rup linux-2.5.69-mm8-clean/include/linux/dcache.h linux-2.5.69-mm8-dcache-count/include/linux/dcache.h --- linux-2.5.69-mm8-clean/include/linux/dcache.h Mon May 19 13:21:34 2003 +++ linux-2.5.69-mm8-dcache-count/include/linux/dcache.h Mon May 19 14:03:23 2003 @@ -37,6 +37,7 @@ struct qstr { struct dentry_stat_t { int nr_dentry; int nr_unused; + atomic_t nr_alloced; int age_limit; /* age in seconds */ int want_pages; /* pages requested by system */ int dummy[2]; diff -rup linux-2.5.69-mm8-clean/include/linux/fs.h linux-2.5.69-mm8-dcache-count/include/linux/fs.h --- linux-2.5.69-mm8-clean/include/linux/fs.h Mon May 19 13:25:54 2003 +++ linux-2.5.69-mm8-dcache-count/include/linux/fs.h Mon May 19 13:50:33 2003 @@ -58,6 +58,7 @@ extern struct files_stat_struct files_st struct inodes_stat_t { int nr_inodes; int nr_unused; + atomic_t nr_alloced; int dummy[5]; }; extern struct inodes_stat_t inodes_stat; diff -rup linux-2.5.69-mm8-clean/mm/mmap.c linux-2.5.69-mm8-dcache-count/mm/mmap.c --- linux-2.5.69-mm8-clean/mm/mmap.c Mon May 19 13:25:55 2003 +++ linux-2.5.69-mm8-dcache-count/mm/mmap.c Mon May 19 16:24:33 2003 @@ -82,16 +82,21 @@ int vm_enough_memory(long pages) free += nr_swap_pages; /* - * The code below doesn't account for free space in the - * inode and dentry slab cache, slab cache fragmentation, - * inodes and dentries which will become freeable under - * VM load, etc. Lets just hope all these (complex) - * factors balance out... + * The code below will overestimate the amount of + * reclaimable slab. Previously, using the .nr_unused + * stat, this number was too low and caused too many + * good allocations to fail. This assumes that every + * dentry/inode allocated in the slab is reclaimable, + * which they probably will be if we get deperate + * enough. + * - Dave Hansen <haveblue@us.ibm.com> */ - free += (dentry_stat.nr_unused * sizeof(struct dentry)) >> - PAGE_SHIFT; - free += (inodes_stat.nr_unused * sizeof(struct inode)) >> - PAGE_SHIFT; + free += (atomic_read(&dentry_stat.nr_alloced) * + sizeof(struct dentry)) >> + PAGE_SHIFT; + free += (atomic_read(&inodes_stat.nr_alloced) * + sizeof(struct inode)) >> + PAGE_SHIFT; /* * Leave the last 3% for root
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