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version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Aug 20, 2025 at 10:35:57AM -0600, Nico Pache wrote: > On Wed, Aug 20, 2025 at 5:22 AM Lorenzo Stoakes > wrote: > > > > On Tue, Aug 19, 2025 at 07:41:54AM -0600, Nico Pache wrote: > > > The khugepaged daemon and madvise_collapse have two different > > > implementations that do almost the same thing. > > > > > > Create collapse_single_pmd to increase code reuse and create an entry > > > point to these two users. > > > > > > Refactor madvise_collapse and collapse_scan_mm_slot to use the new > > > collapse_single_pmd function. This introduces a minor behavioral change > > > that is most likely an undiscovered bug. The current implementation of > > > khugepaged tests collapse_test_exit_or_disable before calling > > > collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse > > > case. By unifying these two callers madvise_collapse now also performs > > > this check. > > > > > > Reviewed-by: Baolin Wang > > > Acked-by: David Hildenbrand > > > Signed-off-by: Nico Pache > > > --- > > > mm/khugepaged.c | 94 ++++++++++++++++++++++++++----------------------- > > > 1 file changed, 49 insertions(+), 45 deletions(-) > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index 0e7bbadf03ee..b7b98aebb670 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > > > @@ -2382,6 +2382,50 @@ static int collapse_scan_file(struct mm_struct *mm, unsigned long addr, > > > return result; > > > } > > > > > > +/* > > > + * Try to collapse a single PMD starting at a PMD aligned addr, and return > > > + * the results. > > > + */ > > > +static int collapse_single_pmd(unsigned long addr, > > > + struct vm_area_struct *vma, bool *mmap_locked, > > > + struct collapse_control *cc) > > > +{ > > > + int result = SCAN_FAIL; > > > > You assign result in all branches, so this can be uninitialised. > ack, thanks. > > > > > + struct mm_struct *mm = vma->vm_mm; > > > + > > > + if (!vma_is_anonymous(vma)) { > > > + struct file *file = get_file(vma->vm_file); > > > + pgoff_t pgoff = linear_page_index(vma, addr); > > > + > > > + mmap_read_unlock(mm); > > > + *mmap_locked = false; > > > + result = collapse_scan_file(mm, addr, file, pgoff, cc); > > > + fput(file); > > > + if (result == SCAN_PTE_MAPPED_HUGEPAGE) { > > > + mmap_read_lock(mm); > > > + *mmap_locked = true; > > > + if (collapse_test_exit_or_disable(mm)) { > > > + mmap_read_unlock(mm); > > > + *mmap_locked = false; > > > + result = SCAN_ANY_PROCESS; > > > + goto end; > > > > Don't love that in e.g. collapse_scan_mm_slot() we are using the mmap lock being > > disabled as in effect an error code. > > > > Is SCAN_ANY_PROCESS correct here? Because in collapse_scan_mm_slot() you'd > > previously: > https://lore.kernel.org/lkml/a881ed65-351a-469f-b625-a3066d0f1d5c@linux.alibaba.com/ > Baolin brought up a good point a while back that if > collapse_test_exit_or_disable returns true we will be breaking out of > the loop and should change the return value to indicate this. So to > combine the madvise breakout and the scan_slot breakout we drop the > lock and return SCAN_ANY_PROCESS. Let's document in commit msg, as Liam's pointed out it's really important to track things, and part of that as well is detailing in the commit message what you're doing + why. With the THP code being as 'organically grown' as it is shall we say :) it's even more mportant to be specific. > > > > if (*result == SCAN_PTE_MAPPED_HUGEPAGE) { > > mmap_read_lock(mm); > > if (collapse_test_exit_or_disable(mm)) > > goto breakouterloop; > > ... > > } > > > > But now you're setting result = SCAN_ANY_PROCESS rather than > > SCAN_PTE_MAPPED_HUGEPAGE in this instance? > > > > You don't mention that you're changing this, or at least explicitly enough, > > the commit message should state that you're changing this and explain why > > it's ok. > I do state it but perhaps I need to be more verbose! I will update the > message to state we are also changing the result value too. Thanks! > > > > This whole file is horrid, and it's kinda an aside, but I really wish we > > had some comment going through each of the scan_result cases and explaining > > what each one meant. > Yeah its been a huge pain to have to investigate what everything is > supposed to mean, and I often have to go searching to confirm things. > include/trace/events/huge_memory.h has a "good" summary of them > > > > Also I think: > > > > return SCAN_ANY_PROCESS; > > > > Is better than: > > > > result = SCAN_ANY_PROCESS; > > goto end; > I agree! I will change that :) > > ... > > end: > > return result; > > > > > + } > > > + result = collapse_pte_mapped_thp(mm, addr, > > > + !cc->is_khugepaged); > > > > Hm another change here, in the original code in collapse_scan_mm_slot() > > this is: > > > > *result = collapse_pte_mapped_thp(mm, > > khugepaged_scan.address, false); > > > > Presumably collapse_scan_mm_slot() is only ever invoked with > > cc->is_khugepaged? > Correct, but the madvise_collapse calls this with true, hence why it > now depends on the is_khugepaged variable. No functional change here. > > > > Maybe worth adding a VM_WARN_ON_ONCE(!cc->is_khugepaged) at the top of > > collapse_scan_mm_slot() to assert this (and other places where your change > > assumes this to be the case). > Ok I will investigate doing that but it would take a huge mistake to > hit that assertion. > > > > > > > + if (result == SCAN_PMD_MAPPED) > > > + result = SCAN_SUCCEED; > > > + mmap_read_unlock(mm); > > > + *mmap_locked = false; > > > + } > > > + } else { > > > + result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc); > > > + } > > > + if (cc->is_khugepaged && result == SCAN_SUCCEED) > > > + ++khugepaged_pages_collapsed; > > > > Similarly, presumably because collapse_scan_mm_slot() only ever invoked > > khugepaged case this didn't have the cc->is_khugepaged check? > Correct, we only increment this when its khugepaged, so we need to > guard it so madvise collapse wont increment this. You know what I'm going to say :) commit message please! > > > > > +end: > > > + return result; > > > +} > > > > There's a LOT of nesting going on here, I think we can simplify this a > > lot. If we make the change I noted above re: returning SCAN_ANY_PROCESS< we > > can move the end label up a bit and avoid a ton of nesting, e.g.: > Ah I like this much more, I will try to implement/test it. > > > > static int collapse_single_pmd(unsigned long addr, > > struct vm_area_struct *vma, bool *mmap_locked, > > struct collapse_control *cc) > > { > > struct mm_struct *mm = vma->vm_mm; > > struct file *file; > > pgoff_t pgoff; > > int result; > > > > if (vma_is_anonymous(vma)) { > > result = collapse_scan_pmd(mm, vma, addr, mmap_locked, cc); > > goto end: > > } > > > > file = get_file(vma->vm_file); > > pgoff = linear_page_index(vma, addr); > > > > mmap_read_unlock(mm); > > *mmap_locked = false; > > result = collapse_scan_file(mm, addr, file, pgoff, cc); > > fput(file); > > if (result != SCAN_PTE_MAPPED_HUGEPAGE) > > goto end; > > > > mmap_read_lock(mm); > > *mmap_locked = true; > > if (collapse_test_exit_or_disable(mm)) { > > mmap_read_unlock(mm); > > *mmap_locked = false; > > return SCAN_ANY_PROCESS; > > } > > result = collapse_pte_mapped_thp(mm, addr, !cc->is_khugepaged); > > if (result == SCAN_PMD_MAPPED) > > result = SCAN_SUCCEED; > > mmap_read_unlock(mm); > > *mmap_locked = false; > > > > end: > > if (cc->is_khugepaged && result == SCAN_SUCCEED) > > ++khugepaged_pages_collapsed; > > > > return result; > > } > > > > (untested, thrown together so do double check!) This suggested refactoring work for you? > > > > > + > > > static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result, > > > struct collapse_control *cc) > > > __releases(&khugepaged_mm_lock) > > > @@ -2455,34 +2499,9 @@ static unsigned int collapse_scan_mm_slot(unsigned int pages, int *result, > > > VM_BUG_ON(khugepaged_scan.address < hstart || > > > khugepaged_scan.address + HPAGE_PMD_SIZE > > > > hend); > > > - if (!vma_is_anonymous(vma)) { > > > - struct file *file = get_file(vma->vm_file); > > > - pgoff_t pgoff = linear_page_index(vma, > > > - khugepaged_scan.address); > > > - > > > - mmap_read_unlock(mm); > > > - mmap_locked = false; > > > - *result = collapse_scan_file(mm, > > > - khugepaged_scan.address, file, pgoff, cc); > > > - fput(file); > > > - if (*result == SCAN_PTE_MAPPED_HUGEPAGE) { > > > - mmap_read_lock(mm); > > > - if (collapse_test_exit_or_disable(mm)) > > > - goto breakouterloop; > > > - *result = collapse_pte_mapped_thp(mm, > > > - khugepaged_scan.address, false); > > > - if (*result == SCAN_PMD_MAPPED) > > > - *result = SCAN_SUCCEED; > > > - mmap_read_unlock(mm); > > > - } > > > - } else { > > > - *result = collapse_scan_pmd(mm, vma, > > > - khugepaged_scan.address, &mmap_locked, cc); > > > - } > > > - > > > - if (*result == SCAN_SUCCEED) > > > - ++khugepaged_pages_collapsed; > > > > > > + *result = collapse_single_pmd(khugepaged_scan.address, > > > + vma, &mmap_locked, cc); > > > /* move to next address */ > > > khugepaged_scan.address += HPAGE_PMD_SIZE; > > > progress += HPAGE_PMD_NR; > > > @@ -2799,34 +2818,19 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > > > mmap_assert_locked(mm); > > > memset(cc->node_load, 0, sizeof(cc->node_load)); > > > nodes_clear(cc->alloc_nmask); > > > - if (!vma_is_anonymous(vma)) { > > > - struct file *file = get_file(vma->vm_file); > > > - pgoff_t pgoff = linear_page_index(vma, addr); > > > > > > - mmap_read_unlock(mm); > > > - mmap_locked = false; > > > - result = collapse_scan_file(mm, addr, file, pgoff, cc); > > > - fput(file); > > > - } else { > > > - result = collapse_scan_pmd(mm, vma, addr, > > > - &mmap_locked, cc); > > > - } > > > + result = collapse_single_pmd(addr, vma, &mmap_locked, cc); > > > + > > > > Ack the fact you noted the behaviour change re: > > collapse_test_exit_or_disable() that seems fine. > > > > > if (!mmap_locked) > > > *lock_dropped = true; > > > > > > -handle_result: > > > switch (result) { > > > case SCAN_SUCCEED: > > > case SCAN_PMD_MAPPED: > > > ++thps; > > > break; > > > - case SCAN_PTE_MAPPED_HUGEPAGE: > > > - BUG_ON(mmap_locked); > > > - mmap_read_lock(mm); > > > - result = collapse_pte_mapped_thp(mm, addr, true); > > > - mmap_read_unlock(mm); > > > - goto handle_result; > > > > One thing that differs with new code her is we filter SCAN_PMD_MAPPED to > > SCAN_SUCCEED. > > > > I was about to say 'but ++thps - is this correct' but now I realise this > > was looping back on itself with a goto to do just that... ugh ye gads. > > > > Anwyay that's fine because it doesn't change anything. > > > > Re: switch statement in general, again would be good to always have each > > scan possibility in switch statements, but perhaps given so many not > > practical :) > > Yeah it may be worth investigating for future changes I have for > khugepaged (including the new switch statement I implement later and > you commented on) Ack yeah this can be one for the future! > > > > (that way the compiler warns on missing a newly added enum val) > > > > > /* Whitelisted set of results where continuing OK */ > > > + case SCAN_PTE_MAPPED_HUGEPAGE: > > > case SCAN_PMD_NULL: > > > case SCAN_PTE_NON_PRESENT: > > > case SCAN_PTE_UFFD_WP: > > > -- > > Thanks for the review :) No probs, to underline as well - the critique is to make sure we get this right, my aim here is to get your series landed in as good a form as possible :) > > -- Nico > > > 2.50.1 > > > > > > Cheers, Lorenzo