On Tue, May 10, 2022 at 2:05 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 10 May 2022 13:32:20 -0700 Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> > The hugepage_vma_check() could be reused by khugepaged_enter() and
> > khugepaged_enter_vma_merge(), but it is static in khugepaged.c.
> > Make it non-static and declare it in khugepaged.h.
> >
> > ..
> >
> > @@ -508,20 +508,13 @@ void __khugepaged_enter(struct mm_struct *mm)
> > void khugepaged_enter_vma_merge(struct vm_area_struct *vma,
> > unsigned long vm_flags)
> > {
> > - unsigned long hstart, hend;
> > -
> > - /*
> > - * khugepaged only supports read-only files for non-shmem files.
> > - * khugepaged does not yet work on special mappings. And
> > - * file-private shmem THP is not supported.
> > - */
> > - if (!hugepage_vma_check(vma, vm_flags))
> > - return;
> > -
> > - hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> > - hend = vma->vm_end & HPAGE_PMD_MASK;
> > - if (hstart < hend)
> > - khugepaged_enter(vma, vm_flags);
> > + if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags) &&
> > + khugepaged_enabled() &&
> > + (((vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK) <
> > + (vma->vm_end & HPAGE_PMD_MASK))) {
>
> Reviewing these bounds-checking tests is so hard :( Can we simplify?
Yeah, I think they can be moved into a helper with a more descriptive name.
>
> > + if (hugepage_vma_check(vma, vm_flags))
> > + __khugepaged_enter(vma->vm_mm);
> > + }
> > }
>
> void khugepaged_enter_vma(struct vm_area_struct *vma,
> unsigned long vm_flags)
> {
> if (test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
> return;
> if (!khugepaged_enabled())
> return;
> if (round_up(vma->vm_start, HPAGE_PMD_SIZE) >=
> (vma->vm_end & HPAGE_PMD_MASK))
> return; /* vma is too small */
> if (!hugepage_vma_check(vma, vm_flags))
> return;
> __khugepaged_enter(vma->vm_mm);
> }
>
>
> Also, it might be slightly faster to have checked MMF_VM_HUGEPAGE
> before khugepaged_enabled(), but it looks odd. And it might be slower,
> too - more pointer chasing.
I think most configurations have always or madvise mode set
(khugepaged_enabled() return true), so having checked MMF_VM_HUGEPAGE
before khugepaged_enabled() seems slightly better, but anyway it
should not have measurable effect IMHO.
>
> I wish someone would document hugepage_vma_check().
I will clean up all the stuff further in a new patchset, for example,
trying to consolidate all the different
hugepage_suitable/enabled/active checks.
