| Message ID | 20211115075522.73795-4-peterx@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | userfaultfd-wp: Support shmem and hugetlbfs | expand |
On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote: > We used to check against none pte and in those cases orig_pte should always be > none pte anyway. Is that always true? From what I can see in handle_pte_fault() orig_pte only gets initialised in the !pmd_none() case so might not be pte_none. > This change prepares us to be able to call do_fault() on !none ptes. For > example, we should allow that to happen for pte marker so that we can restore > information out of the pte markers. > > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > mm/memory.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 04662b010005..d5966d9e24c3 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > vmf->address, &vmf->ptl); > ret = 0; > /* Re-check under ptl */ > - if (likely(pte_none(*vmf->pte))) > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > do_set_pte(vmf, page, vmf->address); > else > ret = VM_FAULT_NOPAGE; >
On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote: > On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote: > > We used to check against none pte and in those cases orig_pte should always be > > none pte anyway. > > Is that always true? From what I can see in handle_pte_fault() orig_pte only > gets initialised in the !pmd_none() case so might not be pte_none. I believe it's true, otherwise I must have overlooked. IMHO it's not "when we set orig_pte" that matters - note that finish_fault() (that this patch modifies) is only called for file-backed memories, and it's only called in do_fault() where the pte is not mapped at all. DAX seems to call it too, but still DAX comes from do_fault() too, afaict. The pte will not be mapped in two cases in handle_pte_fault(): - When pmd_none - When !pmd_none, however if we find that pte_none==true, that's: if (pte_none(vmf->orig_pte)) { pte_unmap(vmf->pte); vmf->pte = NULL; } So when we're already in do_fault(), afaict, orig_pte must be pte_none(). Another side note is that, IIUC pte_none() is a looser check than the pte_val()==0 and it should be arch dependent. Thanks, > > > This change prepares us to be able to call do_fault() on !none ptes. For > > example, we should allow that to happen for pte marker so that we can restore > > information out of the pte markers. > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > --- > > mm/memory.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/memory.c b/mm/memory.c > > index 04662b010005..d5966d9e24c3 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > vmf->address, &vmf->ptl); > > ret = 0; > > /* Re-check under ptl */ > > - if (likely(pte_none(*vmf->pte))) > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > > do_set_pte(vmf, page, vmf->address); > > else > > ret = VM_FAULT_NOPAGE; > > > > > >
On Thu, Dec 16, 2021 at 01:38:33PM +0800, Peter Xu wrote: > On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote: > > On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote: > > > We used to check against none pte and in those cases orig_pte should always be > > > none pte anyway. > > > > Is that always true? From what I can see in handle_pte_fault() orig_pte only > > gets initialised in the !pmd_none() case so might not be pte_none. > > I believe it's true, otherwise I must have overlooked. > > IMHO it's not "when we set orig_pte" that matters - note that finish_fault() > (that this patch modifies) is only called for file-backed memories, and it's > only called in do_fault() where the pte is not mapped at all. > > DAX seems to call it too, but still DAX comes from do_fault() too, afaict. > > The pte will not be mapped in two cases in handle_pte_fault(): > > - When pmd_none > > - When !pmd_none, however if we find that pte_none==true, that's: > > if (pte_none(vmf->orig_pte)) { > pte_unmap(vmf->pte); > vmf->pte = NULL; > } > > So when we're already in do_fault(), afaict, orig_pte must be pte_none(). > Another side note is that, IIUC pte_none() is a looser check than the > pte_val()==0 and it should be arch dependent. So one more thing I forgot to mention... Of course above is based on the fact that orig_pte will be initialized to zero when creating vmf structure, and that's done in __handle_mm_fault(): struct vm_fault vmf = { .vma = vma, .address = address & PAGE_MASK, .flags = flags, .pgoff = linear_page_index(vma, address), .gfp_mask = __get_fault_gfp_mask(vma), }; I'm not sure whether I should explicitly set it to pte_val(0), in most C programs we'll already assume it's a proper reset of orig_pte value in c99 initialization format, but if anyone thinks we should do that explicitly plus some comments I can do that too. > > Thanks, > > > > > > This change prepares us to be able to call do_fault() on !none ptes. For > > > example, we should allow that to happen for pte marker so that we can restore > > > information out of the pte markers. > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > --- > > > mm/memory.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index 04662b010005..d5966d9e24c3 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > > vmf->address, &vmf->ptl); > > > ret = 0; > > > /* Re-check under ptl */ > > > - if (likely(pte_none(*vmf->pte))) > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > > > do_set_pte(vmf, page, vmf->address); > > > else > > > ret = VM_FAULT_NOPAGE; > > > > > > > > > > > > > -- > Peter Xu
On Thursday, 16 December 2021 4:50:47 PM AEDT Peter Xu wrote: > On Thu, Dec 16, 2021 at 01:38:33PM +0800, Peter Xu wrote: > > On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote: > > > On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote: > > > > We used to check against none pte and in those cases orig_pte should always be > > > > none pte anyway. > > > > > > Is that always true? From what I can see in handle_pte_fault() orig_pte only > > > gets initialised in the !pmd_none() case so might not be pte_none. > > > > I believe it's true, otherwise I must have overlooked. > > > > IMHO it's not "when we set orig_pte" that matters - note that finish_fault() > > (that this patch modifies) is only called for file-backed memories, and it's > > only called in do_fault() where the pte is not mapped at all. > > > > DAX seems to call it too, but still DAX comes from do_fault() too, afaict. > > > > The pte will not be mapped in two cases in handle_pte_fault(): > > > > - When pmd_none > > > > - When !pmd_none, however if we find that pte_none==true, that's: > > > > if (pte_none(vmf->orig_pte)) { > > pte_unmap(vmf->pte); > > vmf->pte = NULL; > > } > > > > So when we're already in do_fault(), afaict, orig_pte must be pte_none(). > > Another side note is that, IIUC pte_none() is a looser check than the > > pte_val()==0 and it should be arch dependent. > > So one more thing I forgot to mention... Of course above is based on the fact > that orig_pte will be initialized to zero when creating vmf structure, and > that's done in __handle_mm_fault(): > > struct vm_fault vmf = { > .vma = vma, > .address = address & PAGE_MASK, > .flags = flags, > .pgoff = linear_page_index(vma, address), > .gfp_mask = __get_fault_gfp_mask(vma), > }; > > I'm not sure whether I should explicitly set it to pte_val(0), in most C > programs we'll already assume it's a proper reset of orig_pte value in c99 > initialization format, but if anyone thinks we should do that explicitly plus > some comments I can do that too. Ok, that was really my question. Is: if (likely(pte_none(*vmf->pte))) equivalent to: if (likely(pte_same(*vmf->pte, __pte(0)))) for every architecture? Looking at Xtensa for example suggests it might not be: arch/xtensa/include/asm/pgtable.h: # define pte_none(pte) (pte_val(pte) == (_PAGE_CA_INVALID | _PAGE_USER)) > > > > Thanks, > > > > > > > > > This change prepares us to be able to call do_fault() on !none ptes. For > > > > example, we should allow that to happen for pte marker so that we can restore > > > > information out of the pte markers. > > > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > > --- > > > > mm/memory.c | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > index 04662b010005..d5966d9e24c3 100644 > > > > --- a/mm/memory.c > > > > +++ b/mm/memory.c > > > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > > > vmf->address, &vmf->ptl); > > > > ret = 0; > > > > /* Re-check under ptl */ > > > > - if (likely(pte_none(*vmf->pte))) > > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > > > > do_set_pte(vmf, page, vmf->address); > > > > else > > > > ret = VM_FAULT_NOPAGE; > > > > > > > > > > > > > > > > > > > >
On Thu, Dec 16, 2021 at 05:23:40PM +1100, Alistair Popple wrote: > On Thursday, 16 December 2021 4:50:47 PM AEDT Peter Xu wrote: > > On Thu, Dec 16, 2021 at 01:38:33PM +0800, Peter Xu wrote: > > > On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote: > > > > On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote: > > > > > We used to check against none pte and in those cases orig_pte should always be > > > > > none pte anyway. > > > > > > > > Is that always true? From what I can see in handle_pte_fault() orig_pte only > > > > gets initialised in the !pmd_none() case so might not be pte_none. > > > > > > I believe it's true, otherwise I must have overlooked. > > > > > > IMHO it's not "when we set orig_pte" that matters - note that finish_fault() > > > (that this patch modifies) is only called for file-backed memories, and it's > > > only called in do_fault() where the pte is not mapped at all. > > > > > > DAX seems to call it too, but still DAX comes from do_fault() too, afaict. > > > > > > The pte will not be mapped in two cases in handle_pte_fault(): > > > > > > - When pmd_none > > > > > > - When !pmd_none, however if we find that pte_none==true, that's: > > > > > > if (pte_none(vmf->orig_pte)) { > > > pte_unmap(vmf->pte); > > > vmf->pte = NULL; > > > } > > > > > > So when we're already in do_fault(), afaict, orig_pte must be pte_none(). > > > Another side note is that, IIUC pte_none() is a looser check than the > > > pte_val()==0 and it should be arch dependent. > > > > So one more thing I forgot to mention... Of course above is based on the fact > > that orig_pte will be initialized to zero when creating vmf structure, and > > that's done in __handle_mm_fault(): > > > > struct vm_fault vmf = { > > .vma = vma, > > .address = address & PAGE_MASK, > > .flags = flags, > > .pgoff = linear_page_index(vma, address), > > .gfp_mask = __get_fault_gfp_mask(vma), > > }; > > > > I'm not sure whether I should explicitly set it to pte_val(0), in most C > > programs we'll already assume it's a proper reset of orig_pte value in c99 > > initialization format, but if anyone thinks we should do that explicitly plus > > some comments I can do that too. > > Ok, that was really my question. Is: > > if (likely(pte_none(*vmf->pte))) > > equivalent to: > > if (likely(pte_same(*vmf->pte, __pte(0)))) > > for every architecture? Looking at Xtensa for example suggests it might not be: > > arch/xtensa/include/asm/pgtable.h: > # define pte_none(pte) (pte_val(pte) == (_PAGE_CA_INVALID | _PAGE_USER)) Yes, another good question... I never expected arch that has pte_none(pte_val(0))==false.. but indeed xtensa is one of them. I digged a bit more, s390 seems to be the other one. I wondered how it could have worked - I thought e.g. pte_alloc_one() will always return a pgtable page will all zero-filled, whose allocation should require __GFP_ZERO anyway. But then I quickly noticed that pte_alloc_one() is per-arch too.. That explains, because per-arch can re-initialize the default pte values. S390 re-initializes its pgtable pages in arch/s390/mm/pgalloc.c: unsigned long *page_table_alloc(struct mm_struct *mm) memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE); While similarly xtensa has: #define pte_clear(mm,addr,ptep) \ do { update_pte(ptep, __pte(_PAGE_CA_INVALID | _PAGE_USER)); } while (0) The solution should be simple - I could re-introduce FAULT_FLAG_UFFD_WP. That flag used to exist in older versions, e.g. this is v1 of current patchset where it is defined: https://lore.kernel.org/lkml/20210323004912.35132-6-peterx@redhat.com/ I thought this patch can greatly simplify things but I overlooked the pte_none() check you mentioned. So it seems I have no good choice but add that flag back. There's another alternative is we do pte_clear() on vmf->orig_pte as the new way to initialize it. I believe it should work too for s390 and xtensa. Any preference? Thanks, > > > > > > > Thanks, > > > > > > > > > > > > This change prepares us to be able to call do_fault() on !none ptes. For > > > > > example, we should allow that to happen for pte marker so that we can restore > > > > > information out of the pte markers. > > > > > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > > > --- > > > > > mm/memory.c | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > > index 04662b010005..d5966d9e24c3 100644 > > > > > --- a/mm/memory.c > > > > > +++ b/mm/memory.c > > > > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > > > > vmf->address, &vmf->ptl); > > > > > ret = 0; > > > > > /* Re-check under ptl */ > > > > > - if (likely(pte_none(*vmf->pte))) > > > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > > > > > do_set_pte(vmf, page, vmf->address); > > > > > else > > > > > ret = VM_FAULT_NOPAGE; > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
On Thursday, 16 December 2021 6:06:54 PM AEDT Peter Xu wrote: [...] > I wondered how it could have worked - I thought e.g. pte_alloc_one() will > always return a pgtable page will all zero-filled, whose allocation should > require __GFP_ZERO anyway. But then I quickly noticed that pte_alloc_one() is > per-arch too.. That explains, because per-arch can re-initialize the default > pte values. Yes, I have wondered the same things before as well. It's all a little bit of fun some of this stuff. > I thought this patch can greatly simplify things but I overlooked the > pte_none() check you mentioned. So it seems I have no good choice but add that > flag back. > > There's another alternative is we do pte_clear() on vmf->orig_pte as the new > way to initialize it. I believe it should work too for s390 and xtensa. > > Any preference? I prefer the later approach (initialising to pte_clear) as it seems cleaner, and pte_none(pte_clear()) is true for every architecture afaik. > Thanks, > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > This change prepares us to be able to call do_fault() on !none ptes. For > > > > > > example, we should allow that to happen for pte marker so that we can restore > > > > > > information out of the pte markers. > > > > > > > > > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > > > > > --- > > > > > > mm/memory.c | 2 +- > > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > > > index 04662b010005..d5966d9e24c3 100644 > > > > > > --- a/mm/memory.c > > > > > > +++ b/mm/memory.c > > > > > > @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) > > > > > > vmf->address, &vmf->ptl); > > > > > > ret = 0; > > > > > > /* Re-check under ptl */ > > > > > > - if (likely(pte_none(*vmf->pte))) > > > > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) > > > > > > do_set_pte(vmf, page, vmf->address); > > > > > > else > > > > > > ret = VM_FAULT_NOPAGE; > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
On Thu, Dec 16, 2021 at 06:45:07PM +1100, Alistair Popple wrote: > On Thursday, 16 December 2021 6:06:54 PM AEDT Peter Xu wrote: > > [...] > > > I wondered how it could have worked - I thought e.g. pte_alloc_one() will > > always return a pgtable page will all zero-filled, whose allocation should > > require __GFP_ZERO anyway. But then I quickly noticed that pte_alloc_one() is > > per-arch too.. That explains, because per-arch can re-initialize the default > > pte values. > > Yes, I have wondered the same things before as well. It's all a little bit of > fun some of this stuff. > > > I thought this patch can greatly simplify things but I overlooked the > > pte_none() check you mentioned. So it seems I have no good choice but add that > > flag back. > > > > There's another alternative is we do pte_clear() on vmf->orig_pte as the new > > way to initialize it. I believe it should work too for s390 and xtensa. > > > > Any preference? > > I prefer the later approach (initialising to pte_clear) as it seems cleaner, > and pte_none(pte_clear()) is true for every architecture afaik. Will do.
diff --git a/mm/memory.c b/mm/memory.c index 04662b010005..d5966d9e24c3 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4052,7 +4052,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf) vmf->address, &vmf->ptl); ret = 0; /* Re-check under ptl */ - if (likely(pte_none(*vmf->pte))) + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) do_set_pte(vmf, page, vmf->address); else ret = VM_FAULT_NOPAGE;
We used to check against none pte and in those cases orig_pte should always be none pte anyway. This change prepares us to be able to call do_fault() on !none ptes. For example, we should allow that to happen for pte marker so that we can restore information out of the pte markers. Signed-off-by: Peter Xu <peterx@redhat.com> --- mm/memory.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)