Re: [PATCH v17 1/7] mm: support madvise(MADV_FREE)

From: Michal Hocko
Date: Wed Dec 03 2014 - 05:13:43 EST

On Wed 03-12-14 09:00:26, Minchan Kim wrote:
> On Tue, Dec 02, 2014 at 11:01:25AM +0100, Michal Hocko wrote:
> > On Mon 01-12-14 08:56:52, Minchan Kim wrote:
> > [...]
> > > From 2edd6890f92fa4943ce3c452194479458582d88c Mon Sep 17 00:00:00 2001
> > > From: Minchan Kim <minchan@xxxxxxxxxx>
> > > Date: Mon, 1 Dec 2014 08:53:55 +0900
> > > Subject: [PATCH] madvise.2: Document MADV_FREE
> > >
> > > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > ---
> > > man2/madvise.2 | 13 +++++++++++++
> > > 1 file changed, 13 insertions(+)
> > >
> > > diff --git a/man2/madvise.2 b/man2/madvise.2
> > > index 032ead7..33aa936 100644
> > > --- a/man2/madvise.2
> > > +++ b/man2/madvise.2
> > > @@ -265,6 +265,19 @@ file (see
> > > .BR MADV_DODUMP " (since Linux 3.4)"
> > > Undo the effect of an earlier
> > > +.TP
> > > +.BR MADV_FREE " (since Linux 3.19)"
> > > +Gives the VM system the freedom to free pages, and tells the system that
> > > +information in the specified page range is no longer important.
> > > +This is an efficient way of allowing
> > > +.BR malloc (3)
> >
> > This might be rather misleading. Only some malloc implementations are
> > using this feature (jemalloc, right?). So either be specific about which
> > implementation or do not add it at all.
> Make sense. I don't think it's a good idea to say specific example
> in man page, which is rather arguable and limit the idea.
> >
> > > +to free pages anywhere in the address space, while keeping the address space
> > > +valid. The next time that the page is referenced, the page might be demand
> > > +zeroed, or might contain the data that was there before the MADV_FREE call.
> > > +References made to that address space range will not make the VM system page the
> > > +information back in from backing store until the page is modified again.
> >
> > I am not sure I understand the last sentence. So say I did MADV_FREE and
> > the reclaim has dropped that page. I know that the file backed mappings
> > are not supported yet but assume they were for a second... Now, I do
> > read from that location again what is the result?
> Zero page.

OK, it felt strange at first but now that I am thinking about it some
more it starts making sense. So the semantic is: Either zero page
(disconnected from the backing store) or the original content after
madvise(MADV_FREE). The page gets connected to the backing store after
it gets modified again. If this is the case then the sentence in the man
page makes perfect sense.

What made me confused was that I expected file backed pages would get a
fresh page from the origin but this would be awkward I guess.

> > If we consider anon mappings then the backing store is misleading as
> > well because memory was dropped and so always newly allocated.
> When I read the sentence at first, I thought backing store means swap
> so I don't have any trouble to understand it. But I agree your opinion.
> Target for man page is not a kernel developer but application developer.
> > I would rather drop the whole sentence and rather see an explanation
> > what is the difference between to MADV_DONT_NEED.
> > "
> > Unlike MADV_DONT_NEED the memory is freed lazily e.g. when the VM system
> > is under memory pressure.
> > "
> It's a good idea but I don't think it's enough. At least we should explan
> cancel of delay free logic(ie, write). So, How about this?
> MADV_FREE " (since Linux 3.19)"
> Gives the VM system the freedom to free pages, and tells the system that
> it's okay to free pages if the VM system has reasons(e.g., memory pressure).
> So, it looks like delayed MADV_DONTNEED.
> The next time that the page is referenced, the page might be demand
> zeroed if the VM system freed the page. Otherwise, it might contain the data
> that was there before the MADV_FREE call if the VM system didn't free the page.
> New write in the page after the MADV_FREE call makes the VM system not free
> the page any more.

Dunno, I guess the original content was slightly better. Or the
following wording from UNIX man pages is even more descriptive
Tell the kernel that contents in the specified address range are no
longer important and the range will be overwritten. When there is
demand for memory, the system will free pages associated with the
specified address range. In this instance, the next time a page in the
address range is referenced, it will contain all zeroes. Otherwise,
it will con- tain the data that was there prior to the MADV_FREE
call. References made to the address range will not make the system read
from backing store (swap space) until the page is modified again.

This value cannot be used on mappings that have underlying file objects.

I would just clarify the last sentence with addition
(MAP_PRIVATE|MAP_ANONYMOUS mappings in this implementation). The
difference to MADV_DONTNEED is more complicated now so I wouldn't make
the text even more confusing.

Anyway the confusion started on my end so feel free to stick with the
BSD wording (modulo malloc note which is really confusing as the default
glibc allocator doesn't do that AFAIK).

> It works only with private anonymous pages (see mmap(2)).

Michal Hocko
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