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Date:   Wed, 23 Mar 2022 09:28:37 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     linux-kernel@vger.kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>, vbabka@suse.cz,
        surenb@google.com, stable@vger.kernel.org, sfr@canb.auug.org.au,
        rientjes@google.com, nadav.amit@gmail.com,
        quic_charante@quicinc.com, patches@lists.linux.dev,
        linux-mm@kvack.org, mm-commits@vger.kernel.org,
        torvalds@linux-foundation.org
Subject: Re: [patch 163/227] mm: madvise: skip unmapped vma holes passed to
 process_madvise
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On Tue 22-03-22 17:24:58, Minchan Kim wrote:
> On Tue, Mar 22, 2022 at 02:46:48PM -0700, Andrew Morton wrote:
> > From: Charan Teja Kalla <quic_charante@quicinc.com>
> > Subject: mm: madvise: skip unmapped vma holes passed to process_madvise
> > 
> > The process_madvise() system call is expected to skip holes in vma passed
> > through 'struct iovec' vector list.  But do_madvise, which
> > process_madvise() calls for each vma, returns ENOMEM in case of unmapped
> > holes, despite the VMA is processed.
> > 
> > Thus process_madvise() should treat ENOMEM as expected and consider the
> > VMA passed to as processed and continue processing other vma's in the
> > vector list.  Returning -ENOMEM to user, despite the VMA is processed,
> > will be unable to figure out where to start the next madvise.
> > 
> > Link: https://lkml.kernel.org/r/4f091776142f2ebf7b94018146de72318474e686.1647008754.git.quic_charante@quicinc.com
> 
> I thought it was still under discussion and Charan will post next
> version along with previous patch
> "mm: madvise: return correct bytes advised with process_madvise"
> 
> https://lore.kernel.org/linux-mm/7207b2f5-6b3e-aea4-aa1b-9c6d849abe34@quicinc.com/

Yes, I am not even sure the new semantic is sensible[1]. We should discuss
that and see all the consequences. Changing the semantic of an existing
syscall is always tricky going back and forth is even worse.
-- 
Michal Hocko
SUSE Labs