linux-kernel - Re: [PATCH RFC v2 1/2] mm: Don't skip swap entry even if zap

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Date:   Fri, 21 Jan 2022 11:11:42 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH RFC v2 1/2] mm: Don't skip swap entry even if zap_details
 specified

On Thu, Jan 20, 2022 at 06:32:29PM +0800, Peter Xu wrote:
> > Except that here we have no page to check, so it looks like you'll
> > have to change should_zap_page() to deal with this case too, or just
> > check details->check_mapping directly.
> 
> Yeah I prefer this, as we don't have the page* pointer anyway.
> 
> > Which raises the question again
> > of why I did not just use a boolean flag there originally: aah, I think
> > I've found why.  In those days there was a horrible "optimization", for
> > better performance on some benchmark I guess, which when you read from
> > /dev/zero into a private mapping, would map the zero page there (look
> > up read_zero_pagealigned() and zeromap_page_range() if you dare).  So
> > there was another category of page to be skipped along with the anon
> > COWs, and I didn't want multiple tests in the zap loop, so checking
> > check_mapping against page->mapping did both.  I think nowadays you
> > could do it by checking for PageAnon page (or genuine swap entry)
> > instead.
> 
> It must be PageAnon already, isn't it?

I think I see what you meant now..

I assume the special case is gone, how about I switch zap_mappings back into
a boolean altogether in this patchset?  Thanks,

-- 
Peter Xu