Re: [PATCH] x86,mm: delay TLB flush after clearing accessed bit

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Mar 31, 2014 at 8:34 AM, Rik van Riel <riel@redhat.com> wrote:
>
> However, clearing the accessed bit does not lead to any
> consistency issues, there is no reason to flush the TLB
> immediately. The TLB flush can be deferred until some
> later point in time.

Ugh. I absolutely detest this patch.

If we're going to leave the TLB dirty, then dammit, leave it dirty.
Don't play some half-way games.

Here's the patch you should just try:

 int ptep_clear_flush_young(struct vm_area_struct *vma,
        unsigned long address, pte_t *ptep)
 {
     return ptep_test_and_clear_young(vma, address, ptep);
 }

instead of complicating things.

Rationale: if the working set is so big that we start paging things
out, we sure as hell don't need to worry about TLB flushing. It will
flush itself.

And conversely - if it doesn't flush itself, and something stays
marked as "accessed" in the TLB for a long time even though we've
cleared it in the page tables, we don't care, because clearly there
isn't enough memory pressure for the accessed bit to matter.

                  Linus

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