Re: [PATCH v6 12/16] mm: list_lru: replace linear array with xarray

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Apr 1, 2022 at 6:36 AM NeilBrown <neilb@suse.de> wrote:
>
> On Thu, 31 Mar 2022, Muchun Song wrote:
> >
> > Thanks for your report.  I knew the reason. It is because the following
> > patch in this series was missed upstream.  Could you help me test if it
> > works properly?
> >
> > [v6,06/16] nfs42: use a specific kmem_cache to allocate nfs4_xattr_entry
> >
>
> Thanks for the quick response!  That patch helps, but has a bug.  My
> testing resulted in refcount underflow errors.
>
> Problem is that kref_init() is called in nfs4_xattr_entry_init_once().
> This means that it is initialised to '1' the first time an entry is
> allocated, but it is left as zero the second time.
> I applied:
> --- a/fs/nfs/nfs42xattr.c
> +++ b/fs/nfs/nfs42xattr.c
> @@ -191,6 +191,7 @@ nfs4_xattr_alloc_entry(const char *name, const void *value,
>         entry = kmem_cache_alloc_lru(nfs4_xattr_entry_cachep, lru, gfp);
>         if (!entry)
>                 return NULL;
> +       kref_init(&entry->ref);
>         namep = kstrdup_const(name, gfp);
>         if (!namep && name)
>                 goto free_buf;
> @@ -974,7 +975,6 @@ static void nfs4_xattr_entry_init_once(void *p)
>  {
>         struct nfs4_xattr_entry *entry = p;
>
> -       kref_init(&entry->ref);
>         entry->bucket = NULL;
>         INIT_LIST_HEAD(&entry->lru);
>         INIT_LIST_HEAD(&entry->dispose);
>
> and now it seems to work.
>
> The complete patch that I applied is below.  I haven't reviewed it, just
> tested it.
>   Tested-by: NeilBrown <neilb@suse.de>
>

Thanks for your test.  I have looked at the latest code.
I found the following patch has removed GFP_ACCOUNT
on allocation.

5c60e89e71f8 ("NFSv4.2: Fix up an invalid combination of memory
allocation flags")

But this commit forgot to remove SLAB_ACCOUNT when creating
nfs4_xattr_cache_cachep (I think it is a bug).  So I think the following
patch could work.

diff --git a/fs/nfs/nfs42xattr.c b/fs/nfs/nfs42xattr.c
index ad3405c64b9e..e7b34f7e0614 100644
--- a/fs/nfs/nfs42xattr.c
+++ b/fs/nfs/nfs42xattr.c
@@ -997,7 +997,7 @@ int __init nfs4_xattr_cache_init(void)

        nfs4_xattr_cache_cachep = kmem_cache_create("nfs4_xattr_cache_cache",
            sizeof(struct nfs4_xattr_cache), 0,
-           (SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD|SLAB_ACCOUNT),
+           (SLAB_RECLAIM_ACCOUNT|SLAB_MEM_SPREAD),
            nfs4_xattr_cache_init_once);
        if (nfs4_xattr_cache_cachep == NULL)
                return -ENOMEM;