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

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Message-ID: <3088958.1628775479@warthog.procyon.org.uk>
Date:   Thu, 12 Aug 2021 14:37:59 +0100
From:   David Howells <dhowells@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     dhowells@...hat.com, 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()

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.

David