linux-kernel - Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to process

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Date:   Thu, 17 Mar 2022 09:28:46 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Charan Teja Kalla <quic_charante@...cinc.com>, surenb@...gle.com,
        vbabka@...e.cz, rientjes@...gle.com, sfr@...b.auug.org.au,
        edgararriaga@...gle.com, nadav.amit@...il.com, mhocko@...e.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "# 5 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to
 process_madvise

On Wed, Mar 16, 2022 at 02:29:06PM -0700, Andrew Morton wrote:
> On Wed, 16 Mar 2022 19:49:38 +0530 Charan Teja Kalla <quic_charante@...cinc.com> wrote:
> 
> > > IMO, it's worth to note in man page.
> > > 
> > 
> > Or the current patch for just ENOMEM is sufficient here and we just have
> > to update the man page?
> 
> I think the "On success, process_madvise() returns the number of bytes
> advised" behaviour sounds useful.  But madvise() doesn't do that.
> 
> RETURN VALUE
>        On  success, madvise() returns zero.  On error, it returns -1 and errno
>        is set to indicate the error.
> 
> So why is it desirable in the case of process_madvise()?

Since process_madvise deal with multiple ranges and could fail at one of
them in the middle or pocessing, people could decide where the call
failed and then make a strategy whether they will abort at the point or
continue to hint next addresses. Here, problem of the strategy is API
doesn't return any error vaule if it has processed any bytes so they
would have limitation to decide a policy. That's the limitation for
every vector IO syscalls, unfortunately.

> 
> 
> 
> And why was process_madvise() designed this way?   Or was it
> always simply an error in the manpage?