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

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Message-ID: <20230504230736.GA2651828@dread.disaster.area>
Date:   Fri, 5 May 2023 09:07:36 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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 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.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com