Re: [PATCH v2 3/3] mm: remove superfluous __ClearPageWaiters()

[Yu Zhao]

On Wed, Aug 19, 2020 at 04:06:32PM -0700, Yang Shi wrote:
> On Tue, Aug 18, 2020 at 11:47 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >
> > Presumably __ClearPageWaiters() was added to follow the previously
> > removed __ClearPageActive() pattern.
> >
> > Only flags that are in PAGE_FLAGS_CHECK_AT_FREE needs to be properly
> > cleared because otherwise we think there may be some kind of leak.
> > PG_waiters is not one of those flags and leaving the clearing to
> > PAGE_FLAGS_CHECK_AT_PREP is more appropriate.
> 
> Actually TBH I'm not very keen to this change, it seems the clearing
> is just moved around and the allocation side pays for that instead of
> free side.

I'll assume you are referring to the overhead from clearing
PG_waiters. First of all, there is no overhead -- we should have a
serious talk with the hardware team who makes word-size bitwise AND
more than one instruction. And the clearing is done in
free_pages_prepare(), which has nothing to do with allocations.