Re: [PATCH v5 20/37] mm: fix non-compound multi-order memory accounting in __free_pages

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Mar 13, 2024 at 3:04 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Mar 06, 2024 at 10:24:18AM -0800, Suren Baghdasaryan wrote:
> > When a non-compound multi-order page is freed, it is possible that a
> > speculative reference keeps the page pinned. In this case we free all
> > pages except for the first page, which will be freed later by the last
> > put_page(). However put_page() ignores the order of the page being freed,
> > treating it as a 0-order page. This creates a memory accounting imbalance
> > because the pages freed in __free_pages() do not have their own alloc_tag
> > and their memory was accounted to the first page. To fix this the first
> > page should adjust its allocation size counter when "tail" pages are freed.
>
> It's not "ignored".  It's not available!
>
> Better wording:
>
> However the page passed to put_page() is indisinguishable from an
> order-0 page, so it cannot do the accounting, just as it cannot free
> the subsequent pages.  Do the accounting here, where we free the pages.
>
> (I'm sure further improvements are possible)
>
> > +static inline void pgalloc_tag_sub_bytes(struct alloc_tag *tag, unsigned int order)
> > +{
> > +     if (mem_alloc_profiling_enabled() && tag)
> > +             this_cpu_sub(tag->counters->bytes, PAGE_SIZE << order);
> > +}
>
> This is a terribly named function.  And it's not even good for what we
> want to use it for.
>
> static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
> {
>         if (mem_alloc_profiling_enabled() && tag)
>                 this_cpu_sub(tag->counters->bytes, PAGE_SIZE * nr);
> }
>
> > +++ b/mm/page_alloc.c
> > @@ -4697,12 +4697,21 @@ void __free_pages(struct page *page, unsigned int order)
> >  {
> >       /* get PageHead before we drop reference */
> >       int head = PageHead(page);
> > +     struct alloc_tag *tag = pgalloc_tag_get(page);
> >
> >       if (put_page_testzero(page))
> >               free_the_page(page, order);
> >       else if (!head)
> > -             while (order-- > 0)
> > +             while (order-- > 0) {
> >                       free_the_page(page + (1 << order), order);
> > +                     /*
> > +                      * non-compound multi-order page accounts all allocations
> > +                      * to the first page (just like compound one), therefore
> > +                      * we need to adjust the allocation size of the first
> > +                      * page as its order is ignored when put_page() frees it.
> > +                      */
> > +                     pgalloc_tag_sub_bytes(tag, order);
>
> -       else if (!head
> +       else if (!head) {
> +               pgalloc_tag_sub_pages(1 << order - 1);
>                 while (order-- > 0)
>                         free_the_page(page + (1 << order), order);
> +       }
>
> It doesn't need a comment, it's obvious what you're doing.

All suggestions seem fine to me. I'll adjust the next version accordingly.
Thanks for reviewing and the feedback!

>