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    SubjectRe: [PATCH 10/11] maps3: add /proc/kpagecount and /proc/kpageflags interfaces
    On Mon, Oct 15, 2007 at 04:34:57PM -0700, Dave Hansen wrote:
    > On Mon, 2007-10-15 at 18:11 -0500, Matt Mackall wrote:
    > > > Could we just have /proc/kpagereferenced? Is there a legitimate need
    > > > for other flags to be visible?
    > >
    > > Referenced, dirty, uptodate, lru, active, slab, writeback, reclaim,
    > > and buddy all look like they might be interesting to me from the point
    > > of view of watching what's happening in the VM graphically in
    > > real-time.
    > This is true, but it forces a lot of logic from the kernel to be run in
    > userspace to figure out what is going on. Looking at mainline today:
    > #define PG_reclaim 17 /* To be reclaimed asap */
    > ...
    > #define PG_readahead PG_reclaim /* Reminder to do async read-ahead */
    > All of a sudden, to figure out which flag it actually is, we need to
    > have all of the logic that the kernel does.
    > Does this establish a fixed user<->kernel ABI that will keep us from
    > doing this in the future:
    > -#define PG_slab 7 /* slab debug (Suparna wants this) */
    > +#define PG_slab 14 /* slab debug (Suparna wants this) */
    > Or, even something like this:
    > -#define PageSlab(page) test_bit(PG_slab, &(page)->flags)
    > +#define PageSlab(page) (!PageLRU(page) && !PageHighmem(page))

    Yeah, there are a bunch of flags that aren't mutually exclusive and we
    could probably recover a few.

    > If we actually had several (or even still one file) that exposed this
    > state, independent of the actual content of page->flags, I think we'd be
    > better off. I think that's the difference between a fun, super-useful
    > debugging feature and one that can stay in mainline and have
    > applications stay using it (without breaking) for a long time.
    > The flags you listed are things that I would imagine will always exist,
    > logically. But, we might not always have a specific page flag for pages
    > under writeback or in the buddy list for that matter. PG_buddy isn't
    > that old. Perhaps that would be better abstracted to something like
    > page_in_main_allocator().

    Perhaps we need something like:

    flags = page->flags;
    userflags =
    FLAG_BIT(USER_REFERENCED, flags & PG_referenced) |

    etc. for the flags we want to export. This will let us change to

    FLAG_BIT(USER_SLAB, PageSlab(page)) |

    if we make a virtual slab bit.

    And it shows up in grep.

    Unfortunately, i386 test_bit is an asm inline and not a macro so we
    can't hope for the compiler to fold up a bunch of identity bit
    mappings for us.

    Mathematics is the supreme nostalgia of our time.
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      Last update: 2007-10-16 02:39    [W:0.023 / U:1.700 seconds]
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