Messages in this thread | | | Date | Thu, 13 Apr 2023 12:30:04 +0200 | From | Jan Kara <> | Subject | Re: [PATCH v3 3/8] ext4: use __GFP_NOFAIL if allocating extents_status cannot fail |
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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
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