Re: [PATCH] proc: Avoid costly high-order page allocations when reading proc files

Matthew Wilcox
2025-04-02T17:24:18Z

On Wed, Apr 02, 2025 at 02:24:45PM +0200, Michal Hocko wrote:
> On Wed 02-04-25 22:32:14, Dave Chinner wrote:
> > > > > >+ /*
> > > > > >+ * Use vmalloc if the count is too large to avoid costly high-order page
> > > > > >+ * allocations.
> > > > > >+ */
> > > > > >+ if (count < (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
> > > > > >+ kbuf = kvzalloc(count + 1, GFP_KERNEL);
> > > > >
> > > > > Why not move this check into kvmalloc family?
> > > >
> > > > Hmm should this check really be in kvmalloc family?
> > >
> > > Modifying the existing kvmalloc functions risks performance regressions.
> > > Could we instead introduce a new variant like vkmalloc() (favoring
> > > vmalloc over kmalloc) or kvmalloc_costless()?
> >
> > We should fix kvmalloc() instead of continuing to force
> > subsystems to work around the limitations of kvmalloc().
>
> Agreed!
>
> > Have a look at xlog_kvmalloc() in XFS. It implements a basic
> > fast-fail, no retry high order kmalloc before it falls back to
> > vmalloc by turning off direct reclaim for the kmalloc() call.
> > Hence if the there isn't a high-order page on the free lists ready
> > to allocate, it falls back to vmalloc() immediately.

... but if vmalloc fails, it goes around again! This is exactly why
we don't want filesystems implementing workarounds for MM problems.
What a mess.

> if (size > PAGE_SIZE) {
> flags |= __GFP_NOWARN;
>
> if (!(flags & __GFP_RETRY_MAYFAIL))
> flags |= __GFP_NORETRY;
> + else
> + flags &= ~__GFP_DIRECT_RECLAIM;

I think it might be better to do this:

flags |= __GFP_NOWARN;

if (!(flags & __GFP_RETRY_MAYFAIL))
flags |= __GFP_NORETRY;
+ else if (size > (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
+ flags &= ~__GFP_DIRECT_RECLAIM;

I think it's entirely appropriate for a call to kvmalloc() to do
direct reclaim if it's asking for, say, 16KiB and we don't have any of
those available. Better than exacerbating the fragmentation problem by
allocating 4x4KiB pages, each from different groupings.