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Date: Fri, 23 Nov 2018 09:59:32 -0800
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 1/2] mm: Remove redundant test from find_get_pages_contig
Message-ID: <20181123175932.GV3065@bombadil.infradead.org>
References: <20181122213224.12793-1-willy@infradead.org>
 <20181122213224.12793-2-willy@infradead.org>
 <20181123104732.gvhdqyddbsiq3i42@kshutemo-mobl1>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Hugh Dickins <hughd@google.com>

On Fri, Nov 23, 2018 at 01:47:32PM +0300, Kirill A. Shutemov wrote:
> On Thu, Nov 22, 2018 at 01:32:23PM -0800, Matthew Wilcox wrote:
> > After we establish a reference on the page, we check the pointer continues
> > to be in the correct position in i_pages.  There's no need to check the
> > page->mapping or page->index afterwards; if those can change after we've
> > got the reference, they can change after we return the page to the caller.
> 
> Hm. IIRC, page->mapping can be set to NULL due truncation, but what about
> index? When it can be changed? Truncation doesn't touch it.

I think index can only be changed after the refcount has hit zero and
the page is safely out of the pagecache.  I agree that page->mapping can
be set to NULL after the call to xas_reload() ... but then it can also
happen after the check, so the check isn't really buying us anything
that the xas_reload() call doesn't already check.