On Thu, 3 Mar 2022, Yang Shi wrote: > On Wed, Mar 2, 2022 at 5:43 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > > Migration entries do not contribute to a page's reference count: move > > __split_huge_pmd_locked()'s page_ref_add() into pmd_migration's else > > block (along with the page_count() check - a page is quite likely to > > to have reference count frozen to 0 when a migration entry is found). > > > > This will fix a very rare anonymous memory leak, after a split_huge_pmd() > > raced with an anon split_huge_page() or an anon THP migrate_pages(): since > > the wrongly raised refcount stopped the page (perhaps small, perhaps huge, > > depending on when the race hit) from ever being freed. At first I thought > > there were worse risks, from prematurely unfreezing a frozen page: but now > > think that would only affect page cache pages, which do not come this way > > (except for anonymous pages in swap cache, perhaps). > > Thanks for catching this. I agree there may be anon memory leak due to > bumped refcount. But I don't think it could affect page cache page > since that code (bumping refcount) is never called for page cache page > IIUC. Yes, that's what I meant by unfreezing "would only affect page cache pages, which do not come this way". But then remembered that anonymous pages in swap cache also have non-NULL page_mapping(), and involve freezing in the same way that page cache pages do. Now, offhand I forget the status of hugepage swap, so maybe that's impossible, hence "perhaps". Easier to fix than to think through all the ramifications of not fixing, as usual. > > The patch looks good to me. Reviewed-by: Yang Shi <shy828301@xxxxxxxxx> Thanks, Hugh
