On Wed 22-12-21 10:40:18, Linus Torvalds wrote: > On Wed, Dec 22, 2021 at 8:08 AM Jan Kara <jack@xxxxxxx> wrote: > > > > Well, but O_DIRECT reads must use FOLL_PIN in any case because they modify > > page data (and so we need to detect them both for COW and filesystem needs). > > Well, O_DIRECT reads do, but not necessarily writes. I agree. > And hey, even reads have been dodgy in the past when we didn't really > have the pinning logic - there's been a lot of users that just wanted > it to work for their particular use-case rather than in general and in > all situations.. Yes, but currently a malicious user can take the system down (BUG_ON) or cause DIF/DIX failures if he is nasty and tries hard enough with O_DIRECT reads (practically, the window is small so I haven't really seen a report that I could trace to O_DIRECT reads but in principle the problem is the same as with pinning & dirtying done e.g. by video capture drivers and there we've seen these problem happen). So forcing pinning for O_DIRECT reads is IMO mandatory. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
