linux-kernel - Re: [PATCH v2] mm: support anonymous stable page

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Date:   Tue, 22 Nov 2016 20:43:54 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Minchan Kim <minchan@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Hyeoncheol Lee <cheol.lee@....com>, yjay.kim@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: support anonymous stable page

On Tue, 22 Nov 2016, Minchan Kim wrote:
> On Mon, Nov 21, 2016 at 07:46:28PM -0800, Hugh Dickins wrote:
> > 
> > Andrew might ask if we should Cc stable (haha): I think we agree
> > that it's a defect we've been aware of ever since stable pages were
> > first proposed, but nobody has actually been troubled by it before
> > your async zram development: so, you're right to be fixing it ahead
> > of your zram changes, but we don't see a call for backporting.
> 
> I thought so until I see your comment. However, I checked again
> and found it seems a ancient bug since zram birth.
> swap_writepage unlock the page right before submitting bio while
> it keeps the lock during rw_page operation during bdev_write_page.
> So, if zram_rw_page fails(e.g, -ENOMEM) and then fallback to
> submit_bio in __swap_writepage, the problem can occur.

It's not clear to me why that matters.  If it drives zram mad
to the point of crashing the kernel, yes, that would matter.  But
if it just places incomprehensible or mis-CRCed data on the device,
who cares?  The reused swap page is marked dirty, and nobody should
be reading the stale data back off swap.  If you do resend with a
stable tag, please make clear why it matters.

Hugh

> 
> Hmm, I will resend patchset with zram fix part with marking
> the stable.
> 
> Thanks, Hugh!