On Wed, Mar 20, 2019 at 10:06:18AM +0800, Peter Xu wrote: > The idea comes from a discussion between Linus and Andrea [1]. > > Before this patch we only allow a page fault to retry once. We > achieved this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing > handle_mm_fault() the second time. This was majorly used to avoid > unexpected starvation of the system by looping over forever to handle > the page fault on a single page. However that should hardly happen, > and after all for each code path to return a VM_FAULT_RETRY we'll > first wait for a condition (during which time we should possibly yield > the cpu) to happen before VM_FAULT_RETRY is really returned. > > This patch removes the restriction by keeping the > FAULT_FLAG_ALLOW_RETRY flag when we receive VM_FAULT_RETRY. It means > that the page fault handler now can retry the page fault for multiple > times if necessary without the need to generate another page fault > event. Meanwhile we still keep the FAULT_FLAG_TRIED flag so page > fault handler can still identify whether a page fault is the first > attempt or not. > > Then we'll have these combinations of fault flags (only considering > ALLOW_RETRY flag and TRIED flag): > > - ALLOW_RETRY and !TRIED: this means the page fault allows to > retry, and this is the first try > > - ALLOW_RETRY and TRIED: this means the page fault allows to > retry, and this is not the first try > > - !ALLOW_RETRY and !TRIED: this means the page fault does not allow > to retry at all > > - !ALLOW_RETRY and TRIED: this is forbidden and should never be used > > In existing code we have multiple places that has taken special care > of the first condition above by checking against (fault_flags & > FAULT_FLAG_ALLOW_RETRY). This patch introduces a simple helper to > detect the first retry of a page fault by checking against > both (fault_flags & FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & > FAULT_FLAG_TRIED) because now even the 2nd try will have the > ALLOW_RETRY set, then use that helper in all existing special paths. > One example is in __lock_page_or_retry(), now we'll drop the mmap_sem > only in the first attempt of page fault and we'll keep it in follow up > retries, so old locking behavior will be retained. > > This will be a nice enhancement for current code [2] at the same time > a supporting material for the future userfaultfd-writeprotect work, > since in that work there will always be an explicit userfault > writeprotect retry for protected pages, and if that cannot resolve the > page fault (e.g., when userfaultfd-writeprotect is used in conjunction > with swapped pages) then we'll possibly need a 3rd retry of the page > fault. It might also benefit other potential users who will have > similar requirement like userfault write-protection. > > GUP code is not touched yet and will be covered in follow up patch. > > Please read the thread below for more information. > > [1] https://lkml.org/lkml/2017/11/2/833 > [2] https://lkml.org/lkml/2018/12/30/64 > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx> A minor comment suggestion below but it can be fix in a followup patch. [...] > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 80bb6408fe73..f73dbc4a1957 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -336,16 +336,52 @@ extern unsigned int kobjsize(const void *objp); > */ > extern pgprot_t protection_map[16]; > > +/* > + * About FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED: we can specify whether we > + * would allow page faults to retry by specifying these two fault flags > + * correctly. Currently there can be three legal combinations: > + * > + * (a) ALLOW_RETRY and !TRIED: this means the page fault allows retry, and > + * this is the first try > + * > + * (b) ALLOW_RETRY and TRIED: this means the page fault allows retry, and > + * we've already tried at least once > + * > + * (c) !ALLOW_RETRY and !TRIED: this means the page fault does not allow retry > + * > + * The unlisted combination (!ALLOW_RETRY && TRIED) is illegal and should never > + * be used. Note that page faults can be allowed to retry for multiple times, > + * in which case we'll have an initial fault with flags (a) then later on > + * continuous faults with flags (b). We should always try to detect pending > + * signals before a retry to make sure the continuous page faults can still be > + * interrupted if necessary. > + */ > + > #define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */ > #define FAULT_FLAG_MKWRITE 0x02 /* Fault was mkwrite of existing pte */ > #define FAULT_FLAG_ALLOW_RETRY 0x04 /* Retry fault if blocking */ > #define FAULT_FLAG_RETRY_NOWAIT 0x08 /* Don't drop mmap_sem and wait when retrying */ > #define FAULT_FLAG_KILLABLE 0x10 /* The fault task is in SIGKILL killable region */ > -#define FAULT_FLAG_TRIED 0x20 /* Second try */ > +#define FAULT_FLAG_TRIED 0x20 /* We've tried once */ > #define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */ > #define FAULT_FLAG_REMOTE 0x80 /* faulting for non current tsk/mm */ > #define FAULT_FLAG_INSTRUCTION 0x100 /* The fault was during an instruction fetch */ > > +/* > + * Returns true if the page fault allows retry and this is the first > + * attempt of the fault handling; false otherwise. This is mostly > + * used for places where we want to try to avoid taking the mmap_sem > + * for too long a time when waiting for another condition to change, > + * in which case we can try to be polite to release the mmap_sem in > + * the first round to avoid potential starvation of other processes > + * that would also want the mmap_sem. > + */ You should be using kernel function documentation style above. > +static inline bool fault_flag_allow_retry_first(unsigned int flags) > +{ > + return (flags & FAULT_FLAG_ALLOW_RETRY) && > + (!(flags & FAULT_FLAG_TRIED)); > +} > +
