On Thu, Feb 21, 2019 at 04:56:56PM +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 I have few comments on this one. See below. > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > --- > > arch/alpha/mm/fault.c | 2 +- > arch/arc/mm/fault.c | 1 - > arch/arm/mm/fault.c | 3 --- > arch/arm64/mm/fault.c | 5 ----- > arch/hexagon/mm/vm_fault.c | 1 - > arch/ia64/mm/fault.c | 1 - > arch/m68k/mm/fault.c | 3 --- > arch/microblaze/mm/fault.c | 1 - > arch/mips/mm/fault.c | 1 - > arch/nds32/mm/fault.c | 1 - > arch/nios2/mm/fault.c | 3 --- > arch/openrisc/mm/fault.c | 1 - > arch/parisc/mm/fault.c | 2 -- > arch/powerpc/mm/fault.c | 6 ------ > arch/riscv/mm/fault.c | 5 ----- > arch/s390/mm/fault.c | 5 +---- > arch/sh/mm/fault.c | 1 - > arch/sparc/mm/fault_32.c | 1 - > arch/sparc/mm/fault_64.c | 1 - > arch/um/kernel/trap.c | 1 - > arch/unicore32/mm/fault.c | 6 +----- > arch/x86/mm/fault.c | 2 -- > arch/xtensa/mm/fault.c | 1 - > drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 +++++++++--- > include/linux/mm.h | 12 +++++++++++- > mm/filemap.c | 2 +- > mm/shmem.c | 2 +- > 27 files changed, 25 insertions(+), 57 deletions(-) > [...] > diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c > index 29422eec329d..7d3e96a9a7ab 100644 > --- a/arch/parisc/mm/fault.c > +++ b/arch/parisc/mm/fault.c > @@ -327,8 +327,6 @@ void do_page_fault(struct pt_regs *regs, unsigned long code, > else > current->min_flt++; > if (fault & VM_FAULT_RETRY) { > - flags &= ~FAULT_FLAG_ALLOW_RETRY; Don't you need to also add: flags |= FAULT_FLAG_TRIED; Like other arch. [...] > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 248ff0a28ecd..d842c3e02a50 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1483,9 +1483,7 @@ void do_user_addr_fault(struct pt_regs *regs, > if (unlikely(fault & VM_FAULT_RETRY)) { > bool is_user = flags & FAULT_FLAG_USER; > > - /* Retry at most once */ > if (flags & FAULT_FLAG_ALLOW_RETRY) { > - flags &= ~FAULT_FLAG_ALLOW_RETRY; > flags |= FAULT_FLAG_TRIED; > if (is_user && signal_pending(tsk)) > return; So here you have a change in behavior, it can retry indefinitly for as long as they are no signal. Don't you want so test for FAULT_FLAG_TRIED ? [...] > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 80bb6408fe73..4e11c9639f1b 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -341,11 +341,21 @@ extern pgprot_t protection_map[16]; > #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. > + */ You should add why it returns false if it is not the first try ie to avoid starvation. > +static inline bool fault_flag_allow_retry_first(unsigned int flags) > +{ > + return (flags & FAULT_FLAG_ALLOW_RETRY) && > + (!(flags & FAULT_FLAG_TRIED)); > +} > + > #define FAULT_FLAG_TRACE \ > { FAULT_FLAG_WRITE, "WRITE" }, \ > { FAULT_FLAG_MKWRITE, "MKWRITE" }, \ [...]
