linux-kernel - Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on entire memory range

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Date:   Wed, 25 Nov 2020 15:43:22 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Christian Brauner <christian@...uner.io>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Murray <timmurray@...gle.com>, linux-api@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on
 entire memory range

On Wed, Nov 25, 2020 at 03:23:40PM -0800, Suren Baghdasaryan wrote:
> On Wed, Nov 25, 2020 at 3:13 PM Minchan Kim <minchan@...nel.org> wrote:
> >
> > On Mon, Nov 23, 2020 at 09:39:42PM -0800, Suren Baghdasaryan wrote:
> > > process_madvise requires a vector of address ranges to be provided for
> > > its operations. When an advice should be applied to the entire process,
> > > the caller process has to obtain the list of VMAs of the target process
> > > by reading the /proc/pid/maps or some other way. The cost of this
> > > operation grows linearly with increasing number of VMAs in the target
> > > process. Even constructing the input vector can be non-trivial when
> > > target process has several thousands of VMAs and the syscall is being
> > > issued during high memory pressure period when new allocations for such
> > > a vector would only worsen the situation.
> > > In the case when advice is being applied to the entire memory space of
> > > the target process, this creates an extra overhead.
> > > Add PMADV_FLAG_RANGE flag for process_madvise enabling the caller to
> > > advise a memory range of the target process. For now, to keep it simple,
> > > only the entire process memory range is supported, vec and vlen inputs
> > > in this mode are ignored and can be NULL and 0.
> > > Instead of returning the number of bytes that advice was successfully
> > > applied to, the syscall in this mode returns 0 on success. This is due
> > > to the fact that the number of bytes would not be useful for the caller
> > > that does not know the amount of memory the call is supposed to affect.
> > > Besides, the ssize_t return type can be too small to hold the number of
> > > bytes affected when the operation is applied to a large memory range.
> >
> > Can we just use one element in iovec to indicate entire address rather
> > than using up the reserved flags?
> >
> >         struct iovec {
> >                 .iov_base = NULL,
> >                 .iov_len = (~(size_t)0),
> >         };
> >
> > Furthermore, it would be applied for other syscalls where have support
> > iovec if we agree on it.
> >
> 
> The flag also changes the return value semantics. If we follow your
> suggestion we should also agree that in this mode the return value
> will be 0 on success and negative otherwise instead of the number of
> bytes madvise was applied to.

Well, return value will depends on the each API. If the operation is
desruptive, it should return the right size affected by the API but
would be okay with 0 or error, otherwise.