Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache

[Andrew Morton]

On Thu, 18 Oct 2018 20:46:21 -0400 Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:

> On Thu, Oct 18, 2018 at 04:16:40PM -0700, Mike Kravetz wrote:
> > I was not sure about this, and expected someone could come up with
> > something better.  It just seems there are filesystems like huegtlbfs,
> > where it makes no sense wasting cycles traversing the filesystem.  So,
> > let's not even try.
> > 
> > Hoping someone can come up with a better method than hard coding as
> > I have done above.
> 
> It's not strictly required after marking the pages dirty though. The
> real fix is the other one? Could we just drop the hardcoding and let
> it run after the real fix is applied?
> 
> The performance of drop_caches doesn't seem critical, especially with
> gigapages. tmpfs doesn't seem to be optimized away from drop_caches
> and the gain would be bigger for tmpfs if THP is not enabled in the
> mount, so I'm not sure if we should worry about hugetlbfs first.

I guess so.  I can't immediately see a clean way of expressing this so
perhaps it would need a new BDI_CAP_NO_BACKING_STORE.  Such a
thing hardly seems worthwhile for drop_caches.

And drop_caches really shouldn't be there anyway.  It's a standing
workaround for ongoing suckage in pagecache and metadata reclaim
behaviour :(