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

[Nicholas Piggin]

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 :)