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SubjectRe: [RFC][PATCH 2/5] add SWAP_HAS_CACHE flag to swap_map
On Wed, 27 May 2009 13:02:46 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> > @@ -1067,21 +1113,21 @@ static int try_to_unuse(unsigned int typ
> > }
> >
> > /*
> > - * How could swap count reach 0x7fff when the maximum
> > - * pid is 0x7fff, and there's no way to repeat a swap
> > - * page within an mm (except in shmem, where it's the
> > - * shared object which takes the reference count)?
> > - * We believe SWAP_MAP_MAX cannot occur in Linux 2.4.
> > - *
> > + * How could swap count reach 0x7ffe ?
> > + * There's no way to repeat a swap page within an mm
> > + * (except in shmem, where it's the shared object which takes
> > + * the reference count)?
> > + * We believe SWAP_MAP_MAX cannot occur.(if occur, unsigned
> > + * short is too small....)
> > * If that's wrong, then we should worry more about
> > * exit_mmap() and do_munmap() cases described above:
> > * we might be resetting SWAP_MAP_MAX too early here.
> > * We know "Undead"s can happen, they're okay, so don't
> > * report them; but do report if we reset SWAP_MAP_MAX.
> > */
> > - if (*swap_map == SWAP_MAP_MAX) {
> > + if (swap_count(*swap_map) == SWAP_MAP_MAX) {
> > spin_lock(&swap_lock);
> > - *swap_map = 1;
> > + *swap_map = make_swap_count(0, 1);
> Can we assume the entry has SWAP_HAS_CACHE here ?
> Shouldn't we check PageSwapCache beforehand ?
>

IIUC, in this try_to_unuse code, the page is added to swap cache and locked
before reaches here. But....ah,ok, unuse_mm() may release lock_page() before
reach here. Then...

if (PageSwapCache(page) && swap_count(*swap_map) == SWAP_MAP_MAX)

is right ? (maybe original code, set to "1" is also buggy.)

Thanks,
-Kame


> > spin_unlock(&swap_lock);
> > reset_overflow = 1;
> > }
>
>
> Thanks,
> Daisuke Nishimura.
>



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