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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <03b0ed0c-51af-1e68-350c-19a3b38a6e48@redhat.com>
Date:   Fri, 21 Jan 2022 10:01:17 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "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,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: reuse the unshared swapcache page in do_wp_page

>>
>> I did hack something similar and it solved the problem, but I felt it is
>> a hack. If the thread is scheduled on another core, or if the write fault
>> is triggered by another thread it wouldn’t work.
> 
> Yes, it will not match easily. One question would be how often it would
> help in practice and if it would be worth the price.
> 


I did some more testing and I have to admit that your reproducer is
really good at finding corner cases.

Assume we try to handle LRU as discussed ... what I get is a delta
during the test: ./forceswap 2 100000 1


anon_wp_reuse 920
-> we were able to reuse
anon_wp_copy_count 0
-> we failed the final page_count() == 1 check
anon_wp_copy_count_early 634
-> we failed the early page_count() check considering swapcache and lru
anon_wp_copy_lock 1
-> we failed trylock
anon_wp_copy_lru 19
-> we failed to clear the lru cache reference
anon_wp_copy_writeback 99974
-> we failed to clear the swapcache reference due to concurrent
   writeback
anon_wp_copy_swapcache 0
-> we failed to clear the swapcache reference for other reasons

So, yeah, we mostly always hit writeback in forceswap.c.
reuse_swap_page() would have been able to reuse the page if the swap
backend would have supported concurrent writes during writeback (IIUC,
zswap doesn't).

But I think triggering that case that often really is an oddity about
the test case.

-- 
Thanks,

David / dhildenb