Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse

[David Hildenbrand <david@redhat.com>]

On 13.01.21 09:52, David Hildenbrand wrote:
> On 13.01.21 04:31, Linus Torvalds wrote:
>> On Tue, Jan 12, 2021 at 6:16 PM Matthew Wilcox <willy@infradead.org> wrote:
>>>
>>> The thing about the speculative page cache references is that they can
>>> temporarily bump a refcount on a page which _used_ to be in the page
>>> cache and has now been reallocated as some other kind of page.
>>
>> Oh, and thinking about this made me think we might actually have a
>> serious bug here, and it has nothing what-so-ever to do with COW, GUP,
>> or even the page count itself.
>>
>> It's unlikely enough that I think it's mostly theoretical, but tell me
>> I'm wrong.
>>
>> PLEASE tell me I'm wrong:
>>
>> CPU1 does page_cache_get_speculative under RCU lock
>>
>> CPU2 frees and re-uses the page
>>
>>     CPU1                CPU2
>>     ----                ----
>>
>>     page = xas_load(&xas);
>>     if (!page_cache_get_speculative(page))
>>             goto repeat;
>>     .. succeeds ..
>>
>>                         remove page from XA
>>                         release page
>>                         reuse for something else
>>
>>     .. and then re-check ..
>>     if (unlikely(page != xas_reload(&xas))) {
>>             put_page(page);
>>             goto repeat;
>>     }
>>
>> ok, the above all looks fine. We got the speculative ref, but then we
>> noticed that its' not valid any more, so we put it again. All good,
>> right?
>>
>> Wrong.
>>
>> What if that "reuse for something else" was actually really quick, and
>> both allocated and released it?
>>
>> That still sounds good, right? Yes, now the "put_page()" will be the
>> one that _actually_ releases the page, but we're still fine, right?
>>
>> Very very wrong.
>>
>> The "reuse for something else" on CPU2 might have gotten not an
>> order-0 page, but a *high-order* page. So it allocated (and then
>> immediately free'd) maybe an order-2 allocation with _four_ pages, and
>> the re-use happened when we had coalesced the buddy pages.
>>
>> But when we release the page on CPU1, we will release just _one_ page,
>> and the other three pages will be lost forever.
>>
>> IOW, we restored the page count perfectly fine, but we screwed up the
>> page sizes and buddy information.
>>
>> Ok, so the above is so unlikely from a timing standpoint that I don't
>> think it ever happens, but I don't see why it couldn't happen in
>> theory.
>>
>> Please somebody tell me I'm missing some clever thing we do to make
>> sure this can actually not happen..
> 
> Wasn't that tackled by latest (not merged AFAIKs) __free_pages() changes?
> 
> I'm only able to come up with the doc update, not with the oroginal
> fix/change
> 
> https://lkml.kernel.org/r/20201027025523.3235-1-willy@infradead.org
> 

Sorry, found it, it's in v5.10

commit e320d3012d25b1fb5f3df4edb7bd44a1c362ec10
Author: Matthew Wilcox (Oracle) <willy@infradead.org>
Date:   Tue Oct 13 16:56:04 2020 -0700

    mm/page_alloc.c: fix freeing non-compound pages

and

commit 7f194fbb2dd75e9346b305b8902e177b423b1062
Author: Matthew Wilcox (Oracle) <willy@infradead.org>
Date:   Mon Dec 14 19:11:09 2020 -0800

    mm/page_alloc: add __free_pages() documentation

is in v5.11-rc1

-- 
Thanks,

David / dhildenb