Re: [PATCH 0/4] pagecache scanning with /proc/kpagecache

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, May 22, 2014 at 01:50:22PM +0400, Konstantin Khlebnikov wrote:
> On Thu, May 22, 2014 at 6:33 AM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Wed, 21 May 2014 22:19:55 -0400 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:
> >
> >> > A much nicer interface would be for us to (finally!) implement
> >> > fincore(), perhaps with an enhanced per-present-page payload which
> >> > presents the info which you need (although we don't actually know what
> >> > that info is!).
> >>
> >> page/pfn of each page slot and its page cache tag as shown in patch 4/4.
> >>
> >> > This would require open() - it appears to be a requirement that the
> >> > caller not open the file, but no reason was given for this.
> >> >
> >> > Requiring open() would address some of the obvious security concerns,
> >> > but it will still be possible for processes to poke around and get some
> >> > understanding of the behaviour of other processes.  Careful attention
> >> > should be paid to this aspect of any such patchset.
> >>
> >> Sorry if I missed your point, but this interface defines fixed mapping
> >> between file position in /proc/kpagecache and in-file page offset of
> >> the target file. So we do not need to use seq_file mechanism, that's
> >> why open() is not defined and default one is used.
> >> The same thing is true for /proc/{kpagecount,kpageflags}, from which
> >> I copied/pasted some basic code.
> >
> > I think you did miss my point ;) Please do a web search for fincore -
> > it's a syscall similar to mincore(), only it queries pagecache:
> > fincore(int fd, loff_t offset, ...).  In its simplest form it queries
> > just for present/absent, but we could increase the query payload to
> > incorporate additional per-page info.
> >
> > It would take a lot of thought and discussion to nail down the
> > fincore() interface (we've already tried a couple of times).  But
> > unfortunately, fincore() is probably going to be implemented one day
> > and it will (or at least could) make /proc/kpagecache obsolete.
> >
> 
> It seems fincore() also might obsolete /proc/kpageflags and /proc/pid/pagemap.
> because it might be implemented for /dev/mem and /proc/pid/mem as well
> as for normal files.
> 
> Something like this:
> int fincore(int fd, u64 *kpf, u64 *pfn, size_t length, off_t offset)

As always with new syscalls flags are missing ;)

u64 for kpf doesn't sound future proof enough. What about this:

int fincore(int fd, size_t length, off_t offset,
	unsigned long flags, void *records);

Format of records is defined by what user asks in flags. Like:

 - FINCORE_PFN: records are 64-bit each with pfn;
 - FINCORE_PAGE_FLAGS: records are 64-bit each with flags;
 - FINCORE_PFN | FINCORE_PAGE_FLAGS: records are 128-bit each with pfns
   followed by flags (or vice versa);

New flags can extend the format if we would want to expose more info.

Comments?

BTW, does everybody happy with mincore() interface? We report 1 there if
pte is present, but it doesn't really say much about the page for cases
like zero page...

-- 
 Kirill A. Shutemov

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