On Tue, 15 Feb 2022, Matthew Wilcox wrote: > On Mon, Feb 14, 2022 at 06:26:39PM -0800, Hugh Dickins wrote: > > Add vma argument to mlock_vma_page() and munlock_vma_page(), make them > > inline functions which check (vma->vm_flags & VM_LOCKED) before calling > > mlock_page() and munlock_page() in mm/mlock.c. > > > > Add bool compound to mlock_vma_page() and munlock_vma_page(): this is > > because we have understandable difficulty in accounting pte maps of THPs, > > and if passed a PageHead page, mlock_page() and munlock_page() cannot > > tell whether it's a pmd map to be counted or a pte map to be ignored. > > > [...] > > > > Mlock accounting on THPs has been hard to define, differed between anon > > and file, involved PageDoubleMap in some places and not others, required > > clear_page_mlock() at some points. Keep it simple now: just count the > > pmds and ignore the ptes, there is no reason for ptes to undo pmd mlocks. > > How would you suggest we handle the accounting for folios which are > intermediate in size between PMDs and PTEs? eg, an order-4 page? > Would it make sense to increment mlock_count by HUGE_PMD_NR for > each PMD mapping and by 1 for each PTE mapping? I think you're asking the wrong question here, but perhaps you've already decided there's only one satisfactory answer to the right question. To answer what you've asked: it doesn't matter at all how you count them in mlock_count, just so long as they are counted up and down consistently. Since it's simplest just to count 1 in mlock_count for each pmd or pte, I prefer that (as I did with THPs); but if you prefer to count pmds up and down by HUGE_PMD_NR, that works too. Though, reading again, you're asking about a PMD mapping of an order-4 page? I don't understand how that could be allowed (except on some non-x86 architecture where the page table fits only 16 pages). The question I thought you should be asking is about how to count them in Mlocked. That's tough; but I take it for granted that you would not want per-subpage flags and counts involved (or not unless forced to do so by some regression that turns out to matter). And I think the only satisfactory answer is to count the whole compound_nr() as Mlocked when any part of it (a single pte, a series of ptes, a pmd) is mlocked; and (try to) move folio to Unevictable whenever any part of it is mlocked. That differs from what Kirill decided for THPs (which I cannot confidently describe, but something like count pmd as Mlocked, don't count ptes as Mlocked, but uncount pmd if any ptes), and what I simplified it to in the mm/munlock series (count pmd as Mlocked, ignore ptes), and will tend to show larger numbers for Mlocked than before; but alternatives seem unworkable to me. Hugh
