On 9/17/26 18:22, Lorenzo Stoakes (ARM) wrote: > GUP cannot be used for VMAs which set VMA_IO_BIT - because memory-mapped > I/O must not be accessed on the user's behalf - or VMA_PFNMAP_BIT - because > PFN maps have no folios which the kernel is permitted to access.
The interesting thing will be: what if we want to allow gup for COW'ed pages in PFNMAP. We discussed that already, and it might be a bit harder to squeeze into the new vma[_flags]_can_gup() checks that are also used elsewhere now. Because GUP itself mostly already handles the VMA_PFNMAP_BIT, just has to be taught about vm_normal_page() etc *properly* on all levels. I even have that on my todo list. Updating vma[_flags]_can_gup() won't be enough in that case (ignore VMA_PFNMAP_BIT in cow-mappings), because suddenly we would unlock other code paths that now depend on this helper. Also, there is the secretmem thing I comment on below. I think this also belongs in this helper but other unrelated users don't really want it. Which makes me wonder whether vma[_flags]_can_gup() is actually the right abstraction to use in some other cases that you convert below. For the actual GUP users I think it's the right thing to do. But not for things that are somewhat related to GUP (but actually different) I think you much rather want a separate helper. Long story short: I wonder whether vma[_flags]_can_gup() is the right abstraction, especially for users that don't actually *use* gup but just want slightly similar semantics (no VM_IO|VM_PFNMAP). > > Rather than keeping these checks open-coded, abstract them to > vma_flags_can_gup() and its VMA wrapper vma_can_gup(). > > A number of other places make the same check to decide whether a mapping > can be populated or accessed as GUP would, so update those too. > > While here, drop a reference to 'special' and replace a use of the > deprecated VMA flags API in vma_dump_size(). > > No functional change intended. > > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> > --- > fs/coredump.c | 4 ++-- > include/linux/mm.h | 29 +++++++++++++++++++++++++++++ > mm/gup.c | 7 +++---- > mm/hmm.c | 3 +-- > mm/memory.c | 14 ++++++++------ > mm/mempolicy.c | 3 ++- > 6 files changed, 45 insertions(+), 15 deletions(-) > > diff --git a/fs/coredump.c b/fs/coredump.c > index fb21fb6703dd..9f729c594c47 100644 > --- a/fs/coredump.c > +++ b/fs/coredump.c > @@ -1616,8 +1616,8 @@ static unsigned long vma_dump_size(struct > vm_area_struct *vma, > return 0; > } > > - /* Do not dump I/O mapped devices or special mappings */ > - if (vma->vm_flags & VM_IO) > + /* Do not dump memory-mapped I/O, which may have side effects on read. > */ > + if (vma_test(vma, VMA_IO_BIT)) > return 0; > In the end, we use GUP to lookup the pages through get_dump_page(). I think we should just use the gup helper instead? > /* By default, dump shared memory if mapped from an anonymous file. */ > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 5860a3b4dba9..1249e04d7b98 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1777,6 +1777,35 @@ static inline bool vma_is_persistent(const struct > vm_area_struct *vma) > return vma_flags_is_persistent(&vma->flags); > } > > +/** > + * vma_flags_can_gup() - Do the specified VMA flags permit GUP to access the > + * mapping's pages? > + * @flags: The VMA flags to test. > + * > + * GUP cannot obtain pages from a PFN map (VMA_PFNMAP_BIT), which may have no > + * struct pages behind it, and must not provide access to memory-mapped I/O > + * (VMA_IO_BIT). > + * > + * Returns: true if GUP may access pages from the mapping, otherwise false. > + */ > +static inline bool vma_flags_can_gup(const vma_flags_t *flags) > +{ > + return !vma_flags_test_any(flags, VMA_IO_BIT, VMA_PFNMAP_BIT); > +} This only covers some things though. See check_vma_flags(): vma_is_secretmem() is another case we universally reject and that is just simply incompatible. In the future, it be an address space flag which we can have from the VMA. So we'd want a vma_can_gup() helper but not necessarily a vma_flags_can_gup() helper. I think this belongs into the vma_can_gup() helper. Also, we should better clarify in the doc that other GUP flags will decide whether GUP is actually allowed. the semantics are a bit vague right now "Do the specified VMA flags permit GUP to access". [...] > diff --git a/mm/hmm.c b/mm/hmm.c > index 2f1e98c6b644..e9569b82a1f0 100644 > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -595,8 +595,7 @@ static int hmm_vma_walk_test(unsigned long start, > unsigned long end, > struct hmm_range *range = hmm_vma_walk->range; > struct vm_area_struct *vma = walk->vma; > > - if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)) && > - vma->vm_flags & VM_READ) > + if (vma_can_gup(vma) && vma_test(vma, VMA_READ_BIT)) > return 0; Where do we end up using gup? I don't think we do, because hmm essentially implements an alternative to KVM-style GUP-fast usage. So likely this wants a different helper. > > /* > diff --git a/mm/memory.c b/mm/memory.c > index 6c011979401a..338fce99e711 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2417,11 +2417,11 @@ static bool vm_mixed_zeropage_allowed(struct > vm_area_struct *vma) > * be problematic as soon as the zeropage gets replaced by a different > * page due to vma->vm_ops->pfn_mkwrite, because what's mapped would > * now differ to what GUP looked up. FSDAX is incompatible to > - * FOLL_LONGTERM and VM_IO is incompatible to GUP completely (see > - * check_vma_flags). > + * FOLL_LONGTERM and memory-mapped I/O is incompatible to GUP completely > + * (see vma_can_gup()). > */ > return vma->vm_ops && vma->vm_ops->pfn_mkwrite && > - (vma_is_fsdax(vma) || vma->vm_flags & VM_IO); > + (vma_is_fsdax(vma) || vma_test(vma, VMA_IO_BIT)); This looks a bit misplaces in this patch. Also, not spelled out in the patch description? [...] > retry: > pgdp = pgd_offset(mm, address); > @@ -7316,8 +7317,9 @@ static int __access_remote_vm(struct mm_struct *mm, > unsigned long addr, > } > > /* > - * Check if this is a VM_IO | VM_PFNMAP VMA, which > - * we can access using slightly different code. > + * GUP failed, perhaps because this is a mapping it > + * cannot handle (see vma_can_gup()) - such mappings may > + * provide access via vm_ops->access() instead. > */ > bytes = 0; > #ifdef CONFIG_HAVE_IOREMAP_PROT > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 044ffb4f4128..2fd759e348ca 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2013,7 +2013,8 @@ SYSCALL_DEFINE5(get_mempolicy, int __user *, policy, > > bool vma_migratable(struct vm_area_struct *vma) > { > - if (vma->vm_flags & (VM_IO | VM_PFNMAP)) > + /* Pages which GUP cannot obtain cannot be migrated either. */ > + if (!vma_can_gup(vma)) > return false; It's slightly confusing, because we don't really use GUP (except in one scenario for lookup_node). -- Cheers, David
