Re: [PATCH v3] mm: mincore: use pte_batch_bint() to batch process large folios

[Baolin Wang]

On 2025/5/9 15:51, David Hildenbrand wrote:
> On 09.05.25 02:45, Baolin Wang wrote:
> > When I tested the mincore() syscall, I observed that it takes longer with
> > 64K mTHP enabled on my Arm64 server. The reason is the 
> > mincore_pte_range()
> > still checks each PTE individually, even when the PTEs are contiguous,
> > which is not efficient.
> >
> > Thus we can use pte_batch_hint() to get the batch number of the present
> > contiguous PTEs, which can improve the performance. I tested the 
> > mincore()
> > syscall with 1G anonymous memory populated with 64K mTHP, and observed an
> > obvious performance improvement:
> >
> > w/o patch        w/ patch        changes
> > 6022us            549us            +91%
> >
> > Moreover, I also tested mincore() with disabling mTHP/THP, and did not
> > see any obvious regression for base pages.
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> > Changes from v2:
> > - Re-calculate the max_nr, per Barry.
> > Changes from v1:
> > - Change to use pte_batch_hint() to get the batch number, per Ryan.
> >
> > Note: I observed the min_t() can introduce a slight performance 
> > regression
> > for base pages, so I change to add a batch size check for base pages,
> > which can resolve the performance regression issue.
> > ---
> >   mm/mincore.c | 22 +++++++++++++++++-----
> >   1 file changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/mincore.c b/mm/mincore.c
> > index 832f29f46767..42d6c9c8da86 100644
> > --- a/mm/mincore.c
> > +++ b/mm/mincore.c
> > @@ -21,6 +21,7 @@
> >   #include <linux/uaccess.h>
> >   #include "swap.h"
> > +#include "internal.h"
> >   static int mincore_hugetlb(pte_t *pte, unsigned long hmask, unsigned 
> > long addr,
> >               unsigned long end, struct mm_walk *walk)
> > @@ -105,6 +106,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned 
> > long addr, unsigned long end,
> >       pte_t *ptep;
> >       unsigned char *vec = walk->private;
> >       int nr = (end - addr) >> PAGE_SHIFT;
> > +    int step, i;
> >       ptl = pmd_trans_huge_lock(pmd, vma);
> >       if (ptl) {
> > @@ -118,16 +120,26 @@ static int mincore_pte_range(pmd_t *pmd, 
> > unsigned long addr, unsigned long end,
> >           walk->action = ACTION_AGAIN;
> >           return 0;
> >       }
> > -    for (; addr != end; ptep++, addr += PAGE_SIZE) {
> > +    for (; addr != end; ptep += step, addr += step * PAGE_SIZE) {
> >           pte_t pte = ptep_get(ptep);
> > +        step = 1;
> >           /* We need to do cache lookup too for pte markers */
> >           if (pte_none_mostly(pte))
> >               __mincore_unmapped_range(addr, addr + PAGE_SIZE,
> >                            vma, vec);
> > -        else if (pte_present(pte))
> > -            *vec = 1;
> > -        else { /* pte is a swap entry */
> > +        else if (pte_present(pte)) {
> > +            unsigned int batch = pte_batch_hint(ptep, pte);
> > +
> > +            if (batch > 1) {
> > +                unsigned int max_nr = (end - addr) >> PAGE_SHIFT;
>
> Nit: probably would have called this max_step to match step.

OK. If need respin the patch, I'll rename it.

> > +
> > +                step = min_t(unsigned int, batch, max_nr);
> > +            }
> > +
> > +            for (i = 0; i < step; i++)
> > +                vec[i] = 1;
>
> I'm surprised this micro-optimization matters that much. Probably the 

Me too.

> compiler
> defers the calculation of max_nr. I am not convinced we need that level of
> micro-optimization in this code ...
>
>
> But if we're already micro-optimizing, you could have optimized out the 
> loop as
> well for order-0:
>
>      unsigned int batch = pte_batch_hint(ptep, pte);
>
>      if (batch > 1) {
>          unsigned int max_nr = (end - addr) >> PAGE_SHIFT;
>
>          step = min_t(unsigned int, batch, max_nr);
>          for (i = 0; i < step; i++)
>              vec[i] = 1;
>      } else {
>          *vec = 1;
>      }

I tried this method, and it had no impact on performance.

> In any case
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks.