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

Shakeel Butt
2025-04-02T18:30:20Z

On Wed, Apr 02, 2025 at 06:24:10PM +0100, Matthew Wilcox wrote:
> 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;

The above seems more appropriate then the Michal's bigger hammer.
In addition I think Vlastimil has a very good point about the kswapd
reclaim for such cases (the patch explicitly complains about kcompactd
cpu usage).

>
> 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.