Re: [PATCH] zram: Add a huge_idle writeback mode

Re: [PATCH] zram: Add a huge_idle writeback mode

[Matthew Wilcox]

On Tue, Mar 15, 2022 at 01:34:21PM -0400, Brian Geffon wrote:
> On Tue, Mar 15, 2022 at 1:28 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Tue, Mar 15, 2022 at 10:22:21AM -0700, Brian Geffon wrote:
> > > Today it's only possible to write back as a page, idle, or huge.
> > > A user might want to writeback pages which are huge and idle first
> > > as these idle pages do not require decompression and make a good
> > > first pass for writeback.
> >
> > We're moving towards having many different sizes of page in play,
> > not just PMD and PTE sizes.  Is this patch actually a good idea in
> > a case where we have, eg, a 32kB anonymous page on a system with 4kB
> > pages?  How should zram handle this case?  What's our cut-off for
> > declaring a page to be "huge"?
> >
> 
> Huge isn't a great term IMO, but it is what it is. ZRAM_HUGE is used
> to identify pages which are incompressible. Since zram is a block
> device which presents PAGE_SIZED blocks, do these new changes which
> involve many different page sizes matter as that seems orthogonal to
> the block subsystem. Correct me if I'm misunderstanding.

Oh, so ZRAM's concept of huge is not the same as the "huge" in
"hugetlbfs" or "THP"?  That's not at all confusing ...

Re: [PATCH] zram: Add a huge_idle writeback mode

[Brian Geffon]

On Tue, Mar 15, 2022 at 1:44 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Mar 15, 2022 at 01:34:21PM -0400, Brian Geffon wrote:
> > On Tue, Mar 15, 2022 at 1:28 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Tue, Mar 15, 2022 at 10:22:21AM -0700, Brian Geffon wrote:
> > > > Today it's only possible to write back as a page, idle, or huge.
> > > > A user might want to writeback pages which are huge and idle first
> > > > as these idle pages do not require decompression and make a good
> > > > first pass for writeback.
> > >
> > > We're moving towards having many different sizes of page in play,
> > > not just PMD and PTE sizes.  Is this patch actually a good idea in
> > > a case where we have, eg, a 32kB anonymous page on a system with 4kB
> > > pages?  How should zram handle this case?  What's our cut-off for
> > > declaring a page to be "huge"?
> > >
> >
> > Huge isn't a great term IMO, but it is what it is. ZRAM_HUGE is used
> > to identify pages which are incompressible. Since zram is a block
> > device which presents PAGE_SIZED blocks, do these new changes which
> > involve many different page sizes matter as that seems orthogonal to
> > the block subsystem. Correct me if I'm misunderstanding.
>
> Oh, so ZRAM's concept of huge is not the same as the "huge" in
> "hugetlbfs" or "THP"?  That's not at all confusing ...

I do not disagree, but there isn't much that can be done about it at
this point given the sysfs file takes an argument called "huge"