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

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Date:   Fri, 5 May 2023 12:49:04 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>,
        Ilya Dryomov <idryomov@...il.com>,
        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 Fri 05-05-23 09:07:36, Dave Chinner wrote:
> On Thu, May 04, 2023 at 05:16:48PM +0100, Matthew Wilcox wrote:
> > On Thu, May 04, 2023 at 05:55:56PM +0200, Jan Kara wrote:
> > > 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...
> > 
> > We have a number of flags which don't take effect when modified on a
> > block device with a mounted filesystem on it.  For example, modifying
> > the readahead settings do not change existing files, only new ones.
> > Since this flag is only modifiable for debugging purposes, I think I'm
> > OK with it not affecting already-mounted filesystems.  It feels like a
> > decision that reasonable people could disagree on, though.
> 
> I think an address space flag makes sense, because then we don't
> even have to care about the special bdev sb/inode thing -
> folio->mapping will already point at the bdev mapping and so do the
> right thing.
> 
> That is, if the bdev changes stable_write state, it can toggle the
> AS_STABLE_WRITE flag on it's inode->i_mapping straight away and all
> the folios and files pointing to the bdev mapping will change
> behaviour immediately.  Everything else retains the same behaviour
> we have now - the stable_write state is persistent on the superblock
> until the filesystem mount is cycled.

Yeah, I'm fine with this behavior. I just wasn't sure whether Ilya didn't
need the sysfs change to be visible in the filesystem so that was why I
pointed that out. But apparently he doesn't need it.

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