linux-kernel - Re: [PATCH] mm: reuse the unshared swapcache page in do_wp

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Message-Id: <6225EAFF-B323-4DC5-AC4C-885B29ED7261@gmail.com>
Date:   Thu, 20 Jan 2022 10:11:06 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     "zhangliang (AG)" <zhangliang5@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        wangzhigang17@...wei.com, Matthew Wilcox <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page



> On Jan 20, 2022, at 10:00 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> On 20.01.22 18:48, Nadav Amit wrote:
>> 
>>> On Jan 20, 2022, at 6:15 AM, David Hildenbrand <david@...hat.com> wrote:
>>> 
>>> On 17.01.22 14:31, zhangliang (AG) wrote:
>>>> Sure, I will do that :)
>>> 
>>> I'm polishing up / testing the patches and might send something out for discussion shortly.
>>> Just a note that on my branch was a version with a wrong condition that should have been fixed now.
>>> 
>> 
>> Sorry for being late for the discussion.
>> 
>> David, does any of it regards the lru_cache_add() reference issue that I
>> mentioned? [1]
> 
> No, unfortunately not in that part of my work. *Maybe* we could also try
> to handle that reference similarly to the swapcache, but the question is
> if we can't wait for PageAnonExclusive.
> 
> Right now I have the following in mind to get most parts working as
> exptected:
> 
> 1. Optimize reuse logic for the swapcache as it seems to be easy
> 2. Streamline COW logic and remove reuse_swap_page() -- fix the CVE for
>   THP.
> 3. Introduce PageAnonExclusive and allow FOLL_PIN only on
>   PageAnonExclusive pages.
> 4. Convert O_DIRECT to FOLL_PIN
> 
> We will never ever have to copy a page PageAnonExclusive page in the COW
> handler and can immediately reuse it without even locking the page. The
> existing reuse logic is essentially then used to reset PageAnonExclusive
> on a page (thus it makes sense to work on it) where the flag is not set
> anymore -- or on a fresh page if we have to copy.
> 
> That implies that all these additional references won't care if your app
> doesn't fork() or KSM isn't active. Consequently, anything that
> read-protects anonymous pages will work as expected and should be as
> fast as it gets.
> 
> Sounds good? At least to me. If only swap/migration entries wouldn't be
> harder to handle than I'd wish, that's why it's taking a little and will
> take a little longer.

Thanks for the quick response. I would have to see the logic to set/clear
PageAnonExclusive to fully understand how things are handled.

BTW, I just saw this patch form PeterZ [1] that seems to be related, as
it deals with changing protection on pinned pages.


[1] https://lore.kernel.org/linux-mm/20220120160822.666778608@infradead.org/