linux-kernel - Re: [RFC][PATCH] cifs: Fix cifs_writepages

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Date:   Fri, 24 Feb 2023 20:45:36 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Steve French <stfrench@...rosoft.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Paulo Alcantara <pc@....nz>,
        Huang Ying <ying.huang@...el.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Xin Hao <xhao@...ux.alibaba.com>, linux-mm@...ck.org,
        mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] cifs: Fix cifs_writepages_region()

On Fri, Feb 24, 2023 at 12:16:49PM -0800, Linus Torvalds wrote:
> On Fri, Feb 24, 2023 at 12:14 PM David Howells <dhowells@...hat.com> wrote:
> >
> > Then why do we have to wait for PG_writeback to complete?
> 
> At least for PG_writeback, it's about "the _previous_ dirty write is
> still under way, but - since PG_dirty is set again - the page has been
> dirtied since".
> 
> So we have to start _another_ writeback, because while the current
> writeback *might* have written the updated data, that is not at all
> certain or clear.

also, we only have a writeback bit, not a writeback count.  And when
the current writeback completes, it'll clear that bit.  We're also
being kind to our backing store and not writing to the same block twice
at the same time.

> I'm not sure what the fscache rules are.

My understanding is that the fscache bit is set under several
circumstances, but if the folio is dirty _and_ the fscache bit
is set, it means the folio is currently being written to the cache
device.  I don't see a conflict there; we can write to the backing
store and the cache device at the same time.