linux-kernel - Re: [PATCH] mm: always respect QUEUE_FLAG_STABLE

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Message-ID: <20230504155556.t6byee6shgb27pw5@quack3>
Date:   Thu, 4 May 2023 17:55:56 +0200
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, Ilya Dryomov <idryomov@...il.com>,
        Jan Kara <jack@...e.cz>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: always respect QUEUE_FLAG_STABLE_WRITES on the block
 device

On Thu 04-05-23 15:16:39, Matthew Wilcox wrote:
> On Thu, May 04, 2023 at 03:55:15PM +0200, Christoph Hellwig wrote:
> > On Thu, May 04, 2023 at 12:56:24PM +0200, Ilya Dryomov wrote:
> > > Commit 1cb039f3dc16 ("bdi: replace BDI_CAP_STABLE_WRITES with a queue
> > > and a sb flag") introduced a regression for the raw block device use
> > > case.  Capturing QUEUE_FLAG_STABLE_WRITES flag in set_bdev_super() has
> > > the effect of respecting it only when there is a filesystem mounted on
> > > top of the block device.  If a filesystem is not mounted, block devices
> > > that do integrity checking return sporadic checksum errors.
> > 
> > With "If a file system is not mounted" you want to say "when accessing
> > a block device directly" here, right?  The two are not exclusive..
> > 
> > > Additionally, this commit made the corresponding sysfs knob writeable
> > > for debugging purposes.  However, because QUEUE_FLAG_STABLE_WRITES flag
> > > is captured when the filesystem is mounted and isn't consulted after
> > > that anywhere outside of swap code, changing it doesn't take immediate
> > > effect even though dumping the knob shows the new value.  With no way
> > > to dump SB_I_STABLE_WRITES flag, this is needlessly confusing.
> > 
> > But very much intentional.  s_bdev often is not the only device
> > in a file system, and we should never reference if from core
> > helpers.
> > 
> > So I think we should go with something like this:
> > 
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index db794399900734..aa36cc2a4530c1 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -3129,7 +3129,11 @@ EXPORT_SYMBOL_GPL(folio_wait_writeback_killable);
> >   */
> >  void folio_wait_stable(struct folio *folio)
> >  {
> > -	if (folio_inode(folio)->i_sb->s_iflags & SB_I_STABLE_WRITES)
> > +	struct inode *inode = folio_inode(folio);
> > +	struct super_block *sb = inode->i_sb;
> > +
> > +	if ((sb->s_iflags & SB_I_STABLE_WRITES) ||
> > +	    (sb_is_blkdev_sb(sb) && bdev_stable_writes(I_BDEV(inode))))
> >  		folio_wait_writeback(folio);
> >  }
> >  EXPORT_SYMBOL_GPL(folio_wait_stable);
> 
> I hate both of these patches ;-)  What we should do is add
> AS_STABLE_WRITES, have the appropriate places call
> mapping_set_stable_writes() and then folio_wait_stable() becomes
> 
> 	if (mapping_test_stable_writes(folio->mapping))
> 		folio_wait_writeback(folio);
> 
> and we remove all the dereferences (mapping->host->i_sb->s_iflags, plus
> whatever else is going on there)

For bdev address_space that's easy but what Ilya also mentioned is a
problem when 'stable_write' flag gets toggled on the device and in that
case having to propagate the flag update to all the address_space
structures is a nightmare...

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