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

[Baokun Li]

On 2023/4/13 18:30, Jan Kara wrote:
> 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@xxxxxxx>
> > Signed-off-by: Baokun Li<libaokun1@xxxxxxxxxx>
> 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.

Indeed. GFP_NOFAIL is only applicable to sleepable allocations,

GFP_ATOMIC will ignore it. I didn't notice that.

> 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

Yes, there's really no better way, thank you very much for your review!
I've sent a patch for v4 as you suggested.
Thanks again!

--
With Best Regards,
Baokun Li
.