On 08.02.22 03:05, Pedro Gomes wrote: > On Mon, Feb 7, 2022 at 7:32 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> Just like clearrefs, this can race against GUP-fast to detect pinned >> pages. And just like clearrefs, we're not handling PMDs properly. And >> just like anything that write-protects random anon pages right now, this >> does not consider O_DIRECT as is. >> >> Fortunately, there are not too many users of clearreefs/softdirty >> tracking out there (my search a while ago returned no open source >> users). My assumption is that your feature might see more widespread use. >> >> Adding more random write protection until we fixed the COW issues [1] >> really makes my stomach hurt on a Monday morning. > > I was not aware of these issues. > >> Please, let's defer any more features that rely on write-protecting >> random anon pages until we have ways in place to not corrupt random user >> space. >> >> That is: >> 1) Teaching the COW logic to not copy pages that are pinned -- I'm >> working on that. >> 2) Converting O_DIRECT to use FOLL_PIN instead of FOLL_GET. John is >> working on that. >> >> So I'm not against this change. I'm against this change at this point in >> time. > > I agree. I will wait until the COW problems are solved to send this patch. > > I'll put you on CC once I have something ready to at least handle pinned pages (FOLL_PIN) as expected, to not get them accidentally COWed. -- Thanks, David / dhildenb
