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    SubjectRe: [PATCH 3/3] dcache: account external names as indirectly reclaimable memory
    On Mon, Mar 05, 2018 at 01:37:43PM +0000, Roman Gushchin wrote:
    > I was reported about suspicious growth of unreclaimable slabs
    > on some machines. I've found that it happens on machines
    > with low memory pressure, and these unreclaimable slabs
    > are external names attached to dentries.
    >
    > External names are allocated using generic kmalloc() function,
    > so they are accounted as unreclaimable. But they are held
    > by dentries, which are reclaimable, and they will be reclaimed
    > under the memory pressure.
    >
    > In particular, this breaks MemAvailable calculation, as it
    > doesn't take unreclaimable slabs into account.
    > This leads to a silly situation, when a machine is almost idle,
    > has no memory pressure and therefore has a big dentry cache.
    > And the resulting MemAvailable is too low to start a new workload.
    >
    > To address the issue, the NR_INDIRECTLY_RECLAIMABLE_BYTES counter
    > is used to track the amount of memory, consumed by external names.
    > The counter is increased in the dentry allocation path, if an external
    > name structure is allocated; and it's decreased in the dentry freeing
    > path.
    >
    > To reproduce the problem I've used the following Python script:
    > import os
    >
    > for iter in range (0, 10000000):
    > try:
    > name = ("/some_long_name_%d" % iter) + "_" * 220
    > os.stat(name)
    > except Exception:
    > pass
    >
    > Without this patch:
    > $ cat /proc/meminfo | grep MemAvailable
    > MemAvailable: 7811688 kB
    > $ python indirect.py
    > $ cat /proc/meminfo | grep MemAvailable
    > MemAvailable: 2753052 kB
    >
    > With the patch:
    > $ cat /proc/meminfo | grep MemAvailable
    > MemAvailable: 7809516 kB
    > $ python indirect.py
    > $ cat /proc/meminfo | grep MemAvailable
    > MemAvailable: 7749144 kB
    >
    > Signed-off-by: Roman Gushchin <guro@fb.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    > Cc: Michal Hocko <mhocko@suse.com>
    > Cc: Johannes Weiner <hannes@cmpxchg.org>
    > Cc: linux-fsdevel@vger.kernel.org
    > Cc: linux-kernel@vger.kernel.org
    > Cc: linux-mm@kvack.org
    > Cc: kernel-team@fb.com
    > ---
    > fs/dcache.c | 29 ++++++++++++++++++++++++-----
    > 1 file changed, 24 insertions(+), 5 deletions(-)
    >
    > diff --git a/fs/dcache.c b/fs/dcache.c
    > index 5c7df1df81ff..a0312d73f575 100644
    > --- a/fs/dcache.c
    > +++ b/fs/dcache.c
    > @@ -273,8 +273,16 @@ static void __d_free(struct rcu_head *head)
    > static void __d_free_external(struct rcu_head *head)
    > {
    > struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
    > - kfree(external_name(dentry));
    > - kmem_cache_free(dentry_cache, dentry);
    > + struct external_name *name = external_name(dentry);
    > + unsigned long bytes;
    > +
    > + bytes = dentry->d_name.len + offsetof(struct external_name, name[1]);
    > + mod_node_page_state(page_pgdat(virt_to_page(name)),
    > + NR_INDIRECTLY_RECLAIMABLE_BYTES,
    > + -kmalloc_size(kmalloc_index(bytes)));
    > +
    > + kfree(name);
    > + kmem_cache_free(dentry_cache, dentry);
    > }
    >
    > static inline int dname_external(const struct dentry *dentry)
    > @@ -1598,6 +1606,7 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
    > struct dentry *dentry;
    > char *dname;
    > int err;
    > + size_t reclaimable = 0;
    >
    > dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
    > if (!dentry)
    > @@ -1614,9 +1623,11 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
    > name = &slash_name;
    > dname = dentry->d_iname;
    > } else if (name->len > DNAME_INLINE_LEN-1) {
    > - size_t size = offsetof(struct external_name, name[1]);
    > - struct external_name *p = kmalloc(size + name->len,
    > - GFP_KERNEL_ACCOUNT);
    > + struct external_name *p;
    > +
    > + reclaimable = offsetof(struct external_name, name[1]) +
    > + name->len;
    > + p = kmalloc(reclaimable, GFP_KERNEL_ACCOUNT);

    Can't we use kmem_cache_alloc with own cache created with SLAB_RECLAIM_ACCOUNT
    if they are reclaimable?
    With that, it would help fragmentation problem with __GFP_RECLAIMABLE for
    page allocation as well as counting problem, IMHO.


    > if (!p) {
    > kmem_cache_free(dentry_cache, dentry);
    > return NULL;
    > @@ -1665,6 +1676,14 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
    > }
    > }
    >
    > + if (unlikely(reclaimable)) {
    > + pg_data_t *pgdat;
    > +
    > + pgdat = page_pgdat(virt_to_page(external_name(dentry)));
    > + mod_node_page_state(pgdat, NR_INDIRECTLY_RECLAIMABLE_BYTES,
    > + kmalloc_size(kmalloc_index(reclaimable)));
    > + }
    > +
    > this_cpu_inc(nr_dentry);
    >
    > return dentry;
    > --
    > 2.14.3
    >
    > --
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