Re: [PATCH] fs/select: add vmalloc fallback for select(2)

[Nicholas Piggin]

On Tue, 27 Sep 2016 11:37:24 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> From: Nicholas Piggin
> > Sent: 27 September 2016 12:25
> > On Tue, 27 Sep 2016 10:44:04 +0200
> > Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >   
> > > On 09/23/2016 06:47 PM, Jason Baron wrote:  
> > > > Hi,
> > > >
> > > > On 09/23/2016 03:24 AM, Nicholas Piggin wrote:  
> > > >> On Fri, 23 Sep 2016 14:42:53 +0800
> > > >> "Hillf Danton" <hillf.zj@xxxxxxxxxxxxxxx> wrote:
> > > >>  
> > > >>>>
> > > >>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where size grows
> > > >>>> with the number of fds passed. We had a customer report page allocation
> > > >>>> failures of order-4 for this allocation. This is a costly order, so it might
> > > >>>> easily fail, as the VM expects such allocation to have a lower-order fallback.
> > > >>>>
> > > >>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be
> > > >>>> physically contiguous. Also the allocation is temporary for the duration of the
> > > >>>> syscall, so it's unlikely to stress vmalloc too much.
> > > >>>>
> > > >>>> Note that the poll(2) syscall seems to use a linked list of order-0 pages, so
> > > >>>> it doesn't need this kind of fallback.  
> > > >>
> > > >> How about something like this? (untested)  
> > >
> > > This pushes the limit further, but might just delay the problem. Could be an
> > > optimization on top if there's enough interest, though.  
> > 
> > What's your customer doing with those selects? If they care at all about
> > performance, I doubt they want select to attempt order-4 allocations, fail,
> > then use vmalloc :)  
> 
> If they care about performance they shouldn't be passing select() lists that
> are anywhere near that large.
> If the number of actual fd is small - use poll().

Right. Presumably it's some old app they're still using, no?