linux-kernel - Re: [PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE

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Date:   Tue, 26 Apr 2022 23:10:44 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Peter Collingbourne <pcc@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/highmem: VM_BUG_ON() if offset + len > PAGE_SIZE

On martedì 26 aprile 2022 22:48:11 CEST Andrew Morton wrote:
> On Tue, 26 Apr 2022 22:19:57 +0200 "Fabio M. De Francesco" 
<fmdefrancesco@...il.com> wrote:
> 
> > On martedì 26 aprile 2022 21:34:12 CEST Andrew Morton wrote:
> > > On Tue, 26 Apr 2022 21:30:20 +0200 "Fabio M. De Francesco" 
> > <fmdefrancesco@...il.com> wrote:
> > > 
> > > > Add VM_BUG_ON() bounds checking to make sure that, if "offset + 
len>
> > > > PAGE_SIZE", memset() does not corrupt data in adjacent pages.
> > > > 
> > > 
> > > hm, why?  To match all the other functions in there?
> > > 
> > > 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?
> > > 
> > I am not so sure about it being so useful. I just noted that 
memzero_page() 
> > is the only function of that family that is implemented with no 
> > VM_BUG_ON(). I have no actual proofs of usefulness :( 
> > 
> > This is why yesterday I sent an "RFC Patch" (please see  
> > https://lore.kernel.org/lkml/20220424104806.25396-1-fmdefrancesco@gmail.com/
> > 
> > Soon after sending it I thought that VM_WARN_ON_ONCE() could have been 
> > better suited, but Ira Weiny wrote to use VM_BUG_ON() for consistency.
> > 
> > Now I could either delete all other VM_BUG_ON() or replace them with 
> > VM_WARN_ON_ONCE() (or some other macro). 
> > 
> > Ah, a third solution might be to leave highmem.h as it is now :)
> > 
> > What do you prefer?
> 
> Merge this patch as-is, I guess.  Going through and removing unuseful
> VM_BUG_ON()s is a separable activity.
> 
Thanks!

While at this, I've just noted that you sent a confirmation which lists all 
the patches of mine that are currently in your tree.

I see that you took v1 of "Extend and reorganize Highmem's documentation". 
I suppose that you missed the v2 of that series at
https://lore.kernel.org/lkml/20220425162400.11334-1-fmdefrancesco@gmail.com/ 

Can you please check it?

Thanks,

Fabio