linux-ext4 - Re: [PATCH 2/3] fs/buffer: avoid races with folio migrations on __find_get_block

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Message-ID: <Z-3eDEBFK8md46-7@bombadil.infradead.org>
Date: Wed, 2 Apr 2025 18:02:04 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: brauner@...nel.org, tytso@....edu, adilger.kernel@...ger.ca,
	linux-ext4@...r.kernel.org, riel@...riel.com, willy@...radead.org,
	hannes@...xchg.org, oliver.sang@...el.com, dave@...olabs.net,
	david@...hat.com, axboe@...nel.dk, hare@...e.de,
	david@...morbit.com, djwong@...nel.org, ritesh.list@...il.com,
	linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-mm@...ck.org, gost.dev@...sung.com, p.raghav@...sung.com,
	da.gomez@...sung.com
Subject: Re: [PATCH 2/3] fs/buffer: avoid races with folio migrations on
 __find_get_block_slow()

On Tue, Apr 01, 2025 at 12:57:37PM +0200, Jan Kara wrote:
> On Sat 29-03-25 23:47:31, Luis Chamberlain wrote:
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index c7abb4a029dc..a4e4455a6ce2 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -208,6 +208,15 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
> >  	head = folio_buffers(folio);
> >  	if (!head)
> >  		goto out_unlock;
> > +
> > +	if (folio->mapping->a_ops->migrate_folio &&
> > +	    folio->mapping->a_ops->migrate_folio == buffer_migrate_folio_norefs) {
> 
> This is always true for bdev mapping we have here, isn't it?

Yes, thanks!

> > +		if (folio_test_lru(folio) &&
> 
> Do you expect bdev page cache to contain non-LRU folios? I thought every
> pagecache folio is on LRU so this seems pointless as well? But I may be
> missing something here.
> 
> > +		    folio_test_locked(folio) &&
> > +		    !folio_test_writeback(folio))
> > +			goto out_unlock;
> 
> I find this problematic. It fixes the race with migration, alright
> (although IMO we should have a comment very well explaining the interplay
> of folio lock and mapping->private_lock to make this work - probably in
> buffer_migrate_folio_norefs() - and reference it from here), but there are
> places which expect that if __find_get_block() doesn't return anything,
> this block is not cached in the buffer cache. And your change breaks this
> assumption. 

Yup agreed!

  Luis