linux-kernel - Re: [PATCH v4 19/36] mm: create new codetag references during page splitting

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Message-ID: <CAJuCfpHJoPa_pQNPrcWNZyU7V7=UA4deGFMxh9_aZPyiP0bFSw@mail.gmail.com>
Date: Wed, 28 Feb 2024 10:38:49 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
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Subject: Re: [PATCH v4 19/36] mm: create new codetag references during page splitting

On Wed, Feb 28, 2024 at 10:28 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/28/24 18:50, Suren Baghdasaryan wrote:
> > On Wed, Feb 28, 2024 at 12:47 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >
> >>
> >> Now this might be rare enough that it's not worth fixing if that would be
> >> too complicated, just FYI.
> >
> > Yeah. We can fix this by subtracting the "bytes" counter of the "head"
> > page for all free_the_page(page + (1 << order), order) calls we do
> > inside __free_pages(). But we can't simply use pgalloc_tag_sub()
> > because the "calls" counter will get over-decremented (we allocated
> > all of these pages with one call). I'll need to introduce a new
> > pgalloc_tag_sub_bytes() API and use it here. I feel it's too targeted
> > of a solution but OTOH this is a special situation, so maybe it's
> > acceptable. WDYT?
>
> Hmm I think there's a problem that once you fail put_page_testzero() and
> detect you need to do this, the page might be already gone or reallocated so
> you can't get to the tag for decrementing bytes. You'd have to get it
> upfront (I guess for "head && order > 0" cases) just in case it happens.
> Maybe it's not worth the trouble for such a rare case.

Yes, that hit me when I tried to implement it but there is a simple
solution around that. I can obtain alloc_tag before doing
put_page_testzero() and then decrement bytes counter directly as
needed.
Not sure if it is a rare enough case that we can ignore it but if the
fix is simple enough then might as well do it?

>
> >>
> >>
> >> > Every time
> >> > one of these pages are freed that codetag's "bytes" and "calls"
> >> > counters will be decremented. I think accounting will work correctly
> >> > irrespective of where these pages are freed, in __free_pages() or by
> >> > put_page().
> >> >
> >>
>
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