Re: [PATCH] mm: filemap: clear idle flag for writes

[Yang Shi]

On 6/24/20 12:50 PM, Shakeel Butt wrote:
> On Wed, Jun 24, 2020 at 12:18 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
> > On 6/24/20 11:53 AM, Andrew Morton wrote:
> > > On Thu, 25 Jun 2020 01:43:32 +0800 Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > > Since commit bbddabe2e436aa7869b3ac5248df5c14ddde0cbf ("mm: filemap:
> > > > only do access activations on reads"), mark_page_accessed() is called
> > > > for reads only.  But the idle flag is cleared by mark_page_accessed() so
> > > > the idle flag won't get cleared if the page is write accessed only.
> > > >
> > > > Basically idle page tracking is used to estimate workingset size of
> > > > workload, noticeable size of workingset might be missed if the idle flag
> > > > is not maintained correctly.
> > > >
> > > > It seems good enough to just clear idle flag for write operations.
> > > >
> > > > ...
> > > >
> > > > --- a/mm/filemap.c
> > > > +++ b/mm/filemap.c
> > > > @@ -41,6 +41,7 @@
> > > >    #include <linux/delayacct.h>
> > > >    #include <linux/psi.h>
> > > >    #include <linux/ramfs.h>
> > > > +#include <linux/page_idle.h>
> > > >    #include "internal.h"
> > > >
> > > >    #define CREATE_TRACE_POINTS
> > > > @@ -1630,6 +1631,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
> > > >
> > > >       if (fgp_flags & FGP_ACCESSED)
> > > >               mark_page_accessed(page);
> > > > +    else if (fgp_flags & FGP_WRITE) {
> > > > +            /* Clear idle flag for buffer write */
> > > > +            if (page_is_idle(page))
> > > > +                    clear_page_idle(page);
> > > > +    }
> > > >
> > > >    no_page:
> > > >       if (!page && (fgp_flags & FGP_CREAT)) {
> > > The kerneldoc comment for pagecache_get_page() could do with some
> > > updating - it fails to mention FGP_WRITE, FGP_NOFS and FGP_NOWAIT.
> > Yes, will propose a separate patch later on.
> >
> > > This change seems correct but also will have runtime effects.  What are
> > > they?
> > Other than a couple of extra cycles when idle page tracking is enabled,
> > I didn't think of other effects. It should be negligible. The idle flag
> > doesn't play a role in page reclaim algorithm, so it won't have impact
> > on that.
> The only user visible impact will be on idle page tracking users. They
> will get more accurate data.

Thanks for elaborating this.