Re: [GIT PULL] Two folio fixes for 5.18

[Matthew Wilcox]

On Tue, May 10, 2022 at 03:18:09PM -0700, Andrew Morton wrote:
> On Fri, 6 May 2022 00:43:18 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> >  - Fix readahead creating single-page folios instead of the intended
> >    large folios when doing reads that are not a power of two in size.
> 
> I worry about the idea of using hugepages in readahead.  We're
> increasing the load on the hugepage allocator, which is already
> groaning under the load.

Well, hang on.  We're not using the hugepage allocator, we're using
the page allocator.  We're also using variable order pages, not
necessarily PMD_ORDER.  I was under the impression that we were
using GFP_TRANSHUGE_LIGHT, but I now don't see that.  So that might
be something that needs to be changed.

> The obvious risk is that handing out hugepages to a low-value consumer
> (copying around pagecache which is only ever accessed via the direct
> map) will deny their availability to high-value consumers (that
> compute-intensive task against a large dataset).
> 
> Has testing and instrumentation been used to demonstrate that this is
> not actually going to be a problem, or are we at risk of getting
> unhappy reports?

It's hard to demonstrate that it's definitely not going to cause a
problem.  But I actually believe it will help; by keeping page cache
memory in larger chunks, we make it easier to defrag memory and create
PMD-order pages when they're needed.