Date: Wed, 28 Feb 2024 17:50:50 +0000
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 12:47 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 2/27/24 17:38, Suren Baghdasaryan wrote:
> > On Tue, Feb 27, 2024 at 2:10 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> On 2/21/24 20:40, Suren Baghdasaryan wrote:
> >> > When a high-order page is split into smaller ones, each newly split
> >> > page should get its codetag. The original codetag is reused for these
> >> > pages but it's recorded as 0-byte allocation because original codetag
> >> > already accounts for the original high-order allocated page.
> >>
> >> This was v3 but then you refactored (for the better) so the commit log
> >> could reflect it?
> >
> > Yes, technically mechnism didn't change but I should word it better.
> > Smth like this:
> >
> > When a high-order page is split into smaller ones, each newly split
> > page should get its codetag. After the split each split page will be
> > referencing the original codetag. The codetag's "bytes" counter
> > remains the same because the amount of allocated memory has not
> > changed, however the "calls" counter gets increased to keep the
> > counter correct when these individual pages get freed.
>
> Great, thanks.
> The concern with __free_pages() is not really related to splitting, so for
> this patch:
>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> >
> >>
> >> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> >>
> >> I was going to R-b, but now I recalled the trickiness of
> >> __free_pages() for non-compound pages if it loses the race to a
> >> speculative reference. Will the codetag handling work fine there?
> >
> > I think so. Each non-compoud page has its individual reference to its
> > codetag and will decrement it whenever the page is freed. IIUC the
> > logic in  __free_pages(), when it loses race to a speculative
> > reference it will free all pages except for the first one and the
>
> The "tail" pages of this non-compound high-order page will AFAICS not have
> code tags assigned, so alloc_tag_sub() will be a no-op (or a warning with
> _DEBUG).

Yes, that is correct.

>
> > first one will be freed when the last put_page() happens. If prior to
> > this all these pages were split from one page then all of them will
> > have their own reference which points to the same codetag.
>
> Yeah I'm assuming there's no split before the freeing. This patch about
> splitting just reminded me of that tricky freeing scenario.

Ah, I see. I thought you were talking about a page that was previously split.

>
> So IIUC the "else if (!head)" path of __free_pages() will do nothing about
> the "tail" pages wrt code tags as there are no code tags.
> Then whoever took the speculative "head" page reference will put_page() and
> free it, which will end up in alloc_tag_sub(). This will decrement calls
> properly, but bytes will become imbalanced, because that put_page() will
> pass order-0 worth of bytes - the original order is lost.

Yeah, that's true. put_page() will end up calling
free_unref_page(&folio->page, 0) even if the original order was more
than 0.

>
> 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?

>
>
> > 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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