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

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Message-ID: <CAJuCfpHa_wfcbatEksuEZqWjTxvM0fc_SAdoBf74QYFipA+s7A@mail.gmail.com>
Date: Mon, 19 Aug 2024 19:21:57 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.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 6:46 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 19 Aug 2024 21:40:34 +0100 Matthew Wilcox <willy@...radead.org> wrote:
>
> > On Mon, Aug 19, 2024 at 01:39:16PM -0700, Suren Baghdasaryan wrote:
> > > On Mon, Aug 19, 2024 at 12:34 PM Matthew Wilcox <willy@...radead.org> wrote:
> > > > 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?
> >
> > Because KASAN isn't for production use?
> >
> > > >  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?
> >
> > Fair.
>
> Is a BUILD_BUG_ON() feasible here?

We could, but I didn't think we should prevent people from having such
a configuration if that's what they need...