linux-kernel - Re: [PATCH 5/5] alloc_tag: config to store page allocation tag refs in page flags

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Message-ID: <CAJuCfpH4yFw6RNKVDK0hqXQQhAhMsyGNp5A50E+c2PZd+_vOgw@mail.gmail.com>
Date: Mon, 19 Aug 2024 13:39:16 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, kent.overstreet@...ux.dev, corbet@....net, 
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Subject: Re: [PATCH 5/5] alloc_tag: config to store page allocation tag refs
 in page flags

On Mon, Aug 19, 2024 at 12:34 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Aug 19, 2024 at 08:15:11AM -0700, Suren Baghdasaryan wrote:
> > @@ -91,7 +97,7 @@
> >  #endif
> >
> >  #if ZONES_WIDTH + LRU_GEN_WIDTH + SECTIONS_WIDTH + NODES_WIDTH + \
> > -     KASAN_TAG_WIDTH + LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> > +     KASAN_TAG_WIDTH + ALLOC_TAG_REF_WIDTH + LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> >  #define LAST_CPUPID_WIDTH LAST_CPUPID_SHIFT
> >  #else
> >  #define LAST_CPUPID_WIDTH 0
>
> So if ALLOC_TAG_REF_WIDTH is big enough, it's going to force last_cpupid
> into struct page.

Thanks for taking a look!
Yes, but how is this field different from say KASAN_TAG_WIDTH which
can also force last_cpupid out of page flags?

>  That will misalign struct page and disable HVO -- with no warning!

mminit_verify_pageflags_layout already has a mminit_dprintk() to
indicate this condition. Is that not enough?

>