Messages in this thread | | | Date | Thu, 22 May 2014 13:36:32 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH 0/4] pagecache scanning with /proc/kpagecache |
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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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