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Date:   Tue, 26 Apr 2022 13:48:11 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Ira Weiny <ira.weiny@intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Peter Collingbourne <pcc@google.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE_SIZE
Message-Id: <20220426134811.63a398ea5d2212dfececfd83@linux-foundation.org>
In-Reply-To: <1790890.atdPhlSkOF@leap>
References: <20220426193020.8710-1-fmdefrancesco@gmail.com>
        <20220426123412.51fb58780a8fe2b32c7814ce@linux-foundation.org>
        <1790890.atdPhlSkOF@leap>
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On Tue, 26 Apr 2022 22:19:57 +0200 "Fabio M. De Francesco" <fmdefrancesco@g=
mail.com> wrote:

> On marted=EC 26 aprile 2022 21:34:12 CEST Andrew Morton wrote:
> > On Tue, 26 Apr 2022 21:30:20 +0200 "Fabio M. De Francesco"=20
> <fmdefrancesco@gmail.com> wrote:
> >=20
> > > Add VM_BUG_ON() bounds checking to make sure that, if "offset + len>
> > > PAGE_SIZE", memset() does not corrupt data in adjacent pages.
> > >=20
> >=20
> > hm, why?  To match all the other functions in there?
> >=20
> > I suppose that's logical.  Or we could just delete all the other
> > VM_BUG_ON()s.  Have any of them proven to be at all useful?
> >=20
> I am not so sure about it being so useful. I just noted that memzero_page=
()=20
> is the only function of that family that is implemented with no=20
> VM_BUG_ON(). I have no actual proofs of usefulness :(=20
>=20
> This is why yesterday I sent an "RFC Patch" (please see =20
> https://lore.kernel.org/lkml/20220424104806.25396-1-fmdefrancesco@gmail.c=
om/
>=20
> Soon after sending it I thought that VM_WARN_ON_ONCE() could have been=20
> better suited, but Ira Weiny wrote to use VM_BUG_ON() for consistency.
>=20
> Now I could either delete all other VM_BUG_ON() or replace them with=20
> VM_WARN_ON_ONCE() (or some other macro).=20
>=20
> Ah, a third solution might be to leave highmem.h as it is now :)
>=20
> What do you prefer?

Merge this patch as-is, I guess.  Going through and removing unuseful
VM_BUG_ON()s is a separable activity.