Re: [PATCH v3 3/8] ext4: use __GFP_NOFAIL if allocating extents_status cannot fail

Jan Kara
2023-04-13T10:30:09Z

On Wed 12-04-23 20:41:21, Baokun Li wrote:
> If extent status tree update fails, we have inconsistency between what is
> stored in the extent status tree and what is stored on disk. And that can
> cause even data corruption issues in some cases.
>
> For extents that cannot be dropped we use __GFP_NOFAIL to allocate memory.
> And with the above logic, the undo operation in __es_remove_extent that
> may cause inconsistency if the split extent fails is unnecessary, so we
> remove it as well.
>
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

When I was looking through this patch, I've realized there's a problem with
my plan :-|. See below...

> static struct extent_status *
> ext4_es_alloc_extent(struct inode *inode, ext4_lblk_t lblk, ext4_lblk_t len,
> - ext4_fsblk_t pblk)
> + ext4_fsblk_t pblk, int nofail)
> {
> struct extent_status *es;
> - es = kmem_cache_alloc(ext4_es_cachep, GFP_ATOMIC);
> + gfp_t gfp_flags = GFP_ATOMIC;
> +
> + if (nofail)
> + gfp_flags |= __GFP_NOFAIL;
> +
> + es = kmem_cache_alloc(ext4_es_cachep, gfp_flags);
> if (es == NULL)
> return NULL;

I have remembered that the combination of GFP_ATOMIC and GFP_NOFAIL is
discouraged because the kernel has no sane way of refilling reserves for
atomic allocations when in atomic context. So this combination can result
in lockups.

So what I think we'll have to do is that we'll just have to return error
from __es_insert_extent() and __es_remove_extent() and in the callers we
drop the i_es_lock, allocate needed status entries (one or two depending on
the desired operation) with GFP_KERNEL | GFP_NOFAIL, get the lock again and
pass the preallocated entries into __es_insert_extent /
__es_remove_extent(). It's a bit ugly but we can at least remove those
__es_shrink() calls which are not pretty either.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR