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

[Yang Shi]

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.