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    SubjectRe: [RFC 7/7] mm: madvise support MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER
    On Tue, May 21, 2019 at 11:55:33AM +0900, Minchan Kim wrote:
    > On Mon, May 20, 2019 at 11:28:01AM +0200, Michal Hocko wrote:
    > > [cc linux-api]
    > >
    > > On Mon 20-05-19 12:52:54, Minchan Kim wrote:
    > > > System could have much faster swap device like zRAM. In that case, swapping
    > > > is extremely cheaper than file-IO on the low-end storage.
    > > > In this configuration, userspace could handle different strategy for each
    > > > kinds of vma. IOW, they want to reclaim anonymous pages by MADV_COLD
    > > > while it keeps file-backed pages in inactive LRU by MADV_COOL because
    > > > file IO is more expensive in this case so want to keep them in memory
    > > > until memory pressure happens.
    > > >
    > > > To support such strategy easier, this patch introduces
    > > > MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER options in madvise(2) like
    > > > that /proc/<pid>/clear_refs already has supported same filters.
    > > > They are filters could be Ored with other existing hints using top two bits
    > > > of (int behavior).
    > >
    > > madvise operates on top of ranges and it is quite trivial to do the
    > > filtering from the userspace so why do we need any additional filtering?
    > >
    > > > Once either of them is set, the hint could affect only the interested vma
    > > > either anonymous or file-backed.
    > > >
    > > > With that, user could call a process_madvise syscall simply with a entire
    > > > range(0x0 - 0xFFFFFFFFFFFFFFFF) but either of MADV_ANONYMOUS_FILTER and
    > > > MADV_FILE_FILTER so there is no need to call the syscall range by range.
    > >
    > > OK, so here is the reason you want that. The immediate question is why
    > > cannot the monitor do the filtering from the userspace. Slightly more
    > > work, all right, but less of an API to expose and that itself is a
    > > strong argument against.
    >
    > What I should do if we don't have such filter option is to enumerate all of
    > vma via /proc/<pid>/maps and then parse every ranges and inode from string,
    > which would be painful for 2000+ vmas.

    Just out of curiosity, how do you get to 2000+ distinct memory regions
    in the address space of a mobile app? I'm assuming these aren't files,
    but rather anon objects with poor grouping. Is that from guard pages
    between individual heap allocations or something?

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