linux-kernel - Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use ->direct

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Date:   Thu, 12 Aug 2021 14:50:47 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     trond.myklebust@...marydata.com, darrick.wong@...cle.com,
        hch@....de, jlayton@...nel.org, sfrench@...ba.org,
        torvalds@...ux-foundation.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: Make swap_readpage() for SWP_FS_OPS use
 ->direct_IO() not ->readpage()

On Thu, Aug 12, 2021 at 02:37:59PM +0100, David Howells wrote:
> David Howells <dhowells@...hat.com> wrote:
> 
> > Matthew Wilcox <willy@...radead.org> wrote:
> > 
> > > After submitting the IO here ...
> > > 
> > > > +	if (ret != -EIOCBQUEUED)
> > > > +		swapfile_read_complete(&ki->iocb, ret, 0);
> > > 
> > > We only touch the 'ki' here ... if the caller didn't call read_complete
> > > 
> > > > +	swapfile_put_kiocb(ki);
> > > 
> > > Except for here, which is only touched in order to put the refcount.
> > > 
> > > So why can't swapfile_read_complete() do the work of freeing the ki?
> > 
> > When I was doing something similar for cachefiles, I couldn't get it to work
> > like that.  I'll have another look at that.
> 
> Ah, yes.  generic_file_direct_write() accesses in the kiocb *after* calling
> ->direct_IO(), so the kiocb *must not* go away until after
> generic_file_direct_write() has returned.

This is a read, not a write ... but we don't care about ki_pos being
updated, so that store can be conditioned on IOCB_SWAP being clear.
Or instead of storing directly to ki_pos, we take a pointer to ki_pos
and then redirect that pointer somewhere harmless.