linux-kernel - Re: [PATCH] mm: remove offset check on page->compound

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Message-ID: <YdjfsbAR0UlwyC6b@casper.infradead.org>
Date:   Sat, 8 Jan 2022 00:49:53 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, peterz@...radead.org,
        vbabka@...e.cz, will@...nel.org, linyunsheng@...wei.com,
        aarcange@...hat.com, feng.tang@...el.com, ebiederm@...ssion.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove offset check on page->compound_head and
 folio->lru

On Fri, Jan 07, 2022 at 04:08:25PM -0800, Andrew Morton wrote:
> On Fri, 7 Jan 2022 22:11:20 +0000 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Fri, Jan 07, 2022 at 01:40:59PM +0000, Wei Yang wrote:
> > > On Fri, Jan 07, 2022 at 03:59:01AM +0000, Matthew Wilcox wrote:
> > > >On Thu, Jan 06, 2022 at 11:52:54PM +0000, Wei Yang wrote:
> > > >> FOLIO_MATCH() is used to make sure struct page and folio has identical
> > > >> layout for the first several words.
> > > >> 
> > > >> The comparison of offset between page->compound_head and folio->lru is
> > > >> more like an internal check in struct page.
> > > >> 
> > > >> This patch just removes it.
> > > >> 
> > > >> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
> > > >
> > > >No.
> > > 
> > > Hi, Matthew
> > > 
> > > Would you mind sharing some insight on this check?
> > 
> > It's right there in the comments.
> 
> Well I can't figure out which comment you're referring to?

         * WARNING: bit 0 of the first word is used for PageTail(). That
         * means the other users of this union MUST NOT use the bit to
         * avoid collision and false-positive PageTail().

> > If you can't be bothered to read, why should I write?
> 
> I don't think the punishment comes close to fitting the crime here :(

I felt I had to NACk it as quickly as possible so you didn't apply it.
Otherwise I'd've waited until I was back from holiday before replying
to the earlier patch.  But you applied that lickety-split, so clearly
I'm not allowed to take a week off.