Re: [PATCHv3 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

[Ira Weiny <ira.weiny@intel.com>]

On Tue, Jun 11, 2019 at 08:29:35PM +0800, Pingfan Liu wrote:
> On Fri, Jun 07, 2019 at 02:10:15PM +0800, Pingfan Liu wrote:
> > On Fri, Jun 7, 2019 at 5:17 AM John Hubbard <jhubbard@nvidia.com> wrote:
> > >
> > > On 6/5/19 7:19 PM, Pingfan Liu wrote:
> > > > On Thu, Jun 6, 2019 at 5:49 AM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > ...
> > > >>> --- a/mm/gup.c
> > > >>> +++ b/mm/gup.c
> > > >>> @@ -2196,6 +2196,26 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
> > > >>>       return ret;
> > > >>>  }
> > > >>>
> > > >>> +#ifdef CONFIG_CMA
> > > >>> +static inline int reject_cma_pages(int nr_pinned, struct page **pages)
> > > >>> +{
> > > >>> +     int i;
> > > >>> +
> > > >>> +     for (i = 0; i < nr_pinned; i++)
> > > >>> +             if (is_migrate_cma_page(pages[i])) {
> > > >>> +                     put_user_pages(pages + i, nr_pinned - i);
> > > >>> +                     return i;
> > > >>> +             }
> > > >>> +
> > > >>> +     return nr_pinned;
> > > >>> +}
> > > >>
> > > >> There's no point in inlining this.
> > > > OK, will drop it in V4.
> > > >
> > > >>
> > > >> The code seems inefficient.  If it encounters a single CMA page it can
> > > >> end up discarding a possibly significant number of non-CMA pages.  I
> > > > The trick is the page is not be discarded, in fact, they are still be
> > > > referrenced by pte. We just leave the slow path to pick up the non-CMA
> > > > pages again.
> > > >
> > > >> guess that doesn't matter much, as get_user_pages(FOLL_LONGTERM) is
> > > >> rare.  But could we avoid this (and the second pass across pages[]) by
> > > >> checking for a CMA page within gup_pte_range()?
> > > > It will spread the same logic to hugetlb pte and normal pte. And no
> > > > improvement in performance due to slow path. So I think maybe it is
> > > > not worth.
> > > >
> > > >>
> > >
> > > I think the concern is: for the successful gup_fast case with no CMA
> > > pages, this patch is adding another complete loop through all the
> > > pages. In the fast case.
> > >
> > > If the check were instead done as part of the gup_pte_range(), then
> > > it would be a little more efficient for that case.
> > >
> > > As for whether it's worth it, *probably* this is too small an effect to measure.
> > > But in order to attempt a measurement: running fio (https://github.com/axboe/fio)
> > > with O_DIRECT on an NVMe drive, might shed some light. Here's an fio.conf file
> > > that Jan Kara and Tom Talpey helped me come up with, for related testing:
> > >
> > > [reader]
> > > direct=1
> > > ioengine=libaio
> > > blocksize=4096
> > > size=1g
> > > numjobs=1
> > > rw=read
> > > iodepth=64
> > >
> Unable to get a NVME device to have a test. And when testing fio on the
> tranditional disk, I got the error "fio: engine libaio not loadable
> fio: failed to load engine
> fio: file:ioengines.c:89, func=dlopen, error=libaio: cannot open shared object file: No such file or directory"
> 
> But I found a test case which can be slightly adjusted to met the aim.
> It is tools/testing/selftests/vm/gup_benchmark.c
> 
> Test enviroment:
>   MemTotal:       264079324 kB
>   MemFree:        262306788 kB
>   CmaTotal:              0 kB
>   CmaFree:               0 kB
>   on AMD EPYC 7601
> 
> Test command:
>   gup_benchmark -r 100 -n 64
>   gup_benchmark -r 100 -n 64 -l
> where -r stands for repeat times, -n is nr_pages param for
> get_user_pages_fast(), -l is a new option to test FOLL_LONGTERM in fast
> path, see a patch at the tail.

Thanks!  That is a good test to add.  You should add the patch to the series.

> 
> Test result:
> w/o     477.800000
> w/o-l   481.070000
> a       481.800000
> a-l     640.410000
> b       466.240000  (question a: b outperforms w/o ?)
> b-l     529.740000
> 
> Where w/o is baseline without any patch using v5.2-rc2, a is this series, b
> does the check in gup_pte_range(). '-l' means FOLL_LONGTERM.
> 
> I am suprised that b-l has about 17% improvement than a. (640.41 -529.74)/640.41

Wow that is bigger than I would have thought.  I suspect it gets worse as -n
increases?

>
> As for "question a: b outperforms w/o ?", I can not figure out why, maybe it can be
> considered as variance.

:-/

Does this change with larger -r or -n values?

> 
> Based on the above result, I think it is better to do the check inside
> gup_pte_range().
> 
> Any comment?

I agree.

Ira

> 
> Thanks,
> 
> 
> > Yeah, agreed. Data is more persuasive. Thanks for your suggestion. I
> > will try to bring out the result.
> > 
> > Thanks,
> >   Pingfan
> > 
> 

> ---
> Patch to do check inside gup_pte_range()
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 2ce3091..ba213a0 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1757,6 +1757,10 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  		page = pte_page(pte);
>  
> +		if (unlikely(flags & FOLL_LONGTERM) &&
> +			is_migrate_cma_page(page))
> +				goto pte_unmap;
> +
>  		head = try_get_compound_head(page, 1);
>  		if (!head)
>  			goto pte_unmap;
> @@ -1900,6 +1904,12 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +
>  	head = try_get_compound_head(pmd_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;
> @@ -1941,6 +1951,12 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +
>  	head = try_get_compound_head(pud_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;
> @@ -1978,6 +1994,12 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> +	if (unlikely(flags & FOLL_LONGTERM) &&
> +		is_migrate_cma_page(page)) {
> +		*nr -= refs;
> +		return 0;
> +	}
> +
>  	head = try_get_compound_head(pgd_page(orig), refs);
>  	if (!head) {
>  		*nr -= refs;

> ---
> Patch for testing
> 
> diff --git a/mm/gup_benchmark.c b/mm/gup_benchmark.c
> index 7dd602d..61dec5f 100644
> --- a/mm/gup_benchmark.c
> +++ b/mm/gup_benchmark.c
> @@ -6,8 +6,9 @@
>  #include <linux/debugfs.h>
>  
>  #define GUP_FAST_BENCHMARK	_IOWR('g', 1, struct gup_benchmark)
> -#define GUP_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
> -#define GUP_BENCHMARK		_IOWR('g', 3, struct gup_benchmark)
> +#define GUP_FAST_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
> +#define GUP_LONGTERM_BENCHMARK	_IOWR('g', 3, struct gup_benchmark)
> +#define GUP_BENCHMARK		_IOWR('g', 4, struct gup_benchmark)
>  
>  struct gup_benchmark {
>  	__u64 get_delta_usec;
> @@ -53,6 +54,11 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
>  			nr = get_user_pages_fast(addr, nr, gup->flags & 1,
>  						 pages + i);
>  			break;
> +		case GUP_FAST_LONGTERM_BENCHMARK:
> +			nr = get_user_pages_fast(addr, nr,
> +						 (gup->flags & 1) | FOLL_LONGTERM,
> +						 pages + i);
> +			break;
>  		case GUP_LONGTERM_BENCHMARK:
>  			nr = get_user_pages(addr, nr,
>  					    (gup->flags & 1) | FOLL_LONGTERM,
> @@ -96,6 +102,7 @@ static long gup_benchmark_ioctl(struct file *filep, unsigned int cmd,
>  
>  	switch (cmd) {
>  	case GUP_FAST_BENCHMARK:
> +	case GUP_FAST_LONGTERM_BENCHMARK:
>  	case GUP_LONGTERM_BENCHMARK:
>  	case GUP_BENCHMARK:
>  		break;
> diff --git a/tools/testing/selftests/vm/gup_benchmark.c b/tools/testing/selftests/vm/gup_benchmark.c
> index c0534e2..ade8acb 100644
> --- a/tools/testing/selftests/vm/gup_benchmark.c
> +++ b/tools/testing/selftests/vm/gup_benchmark.c
> @@ -15,8 +15,9 @@
>  #define PAGE_SIZE sysconf(_SC_PAGESIZE)
>  
>  #define GUP_FAST_BENCHMARK	_IOWR('g', 1, struct gup_benchmark)
> -#define GUP_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
> -#define GUP_BENCHMARK		_IOWR('g', 3, struct gup_benchmark)
> +#define GUP_FAST_LONGTERM_BENCHMARK	_IOWR('g', 2, struct gup_benchmark)
> +#define GUP_LONGTERM_BENCHMARK	_IOWR('g', 3, struct gup_benchmark)
> +#define GUP_BENCHMARK		_IOWR('g', 4, struct gup_benchmark)
>  
>  struct gup_benchmark {
>  	__u64 get_delta_usec;
> @@ -37,7 +38,7 @@ int main(int argc, char **argv)
>  	char *file = "/dev/zero";
>  	char *p;
>  
> -	while ((opt = getopt(argc, argv, "m:r:n:f:tTLUSH")) != -1) {
> +	while ((opt = getopt(argc, argv, "m:r:n:f:tTlLUSH")) != -1) {
>  		switch (opt) {
>  		case 'm':
>  			size = atoi(optarg) * MB;
> @@ -54,6 +55,9 @@ int main(int argc, char **argv)
>  		case 'T':
>  			thp = 0;
>  			break;
> +		case 'l':
> +			cmd = GUP_FAST_LONGTERM_BENCHMARK;
> +			break;
>  		case 'L':
>  			cmd = GUP_LONGTERM_BENCHMARK;
>  			break;
> -- 
> 2.7.5
>