Hi, Following the discussions on the first series, https://lore.kernel.org/r/20211124192024.2408218-1-catalin.marinas@xxxxxxx this new patchset aims to generalise the sub-page probing and introduce a minimum size to the fault_in_*() functions. I called this 'v2' but I can rebase it on top of v1 and keep v1 as a btrfs live-lock back-portable fix. The fault_in_*() API improvements would be a new series. Anyway, I'd first like to know whether this is heading in the right direction and whether it's worth adding min_size to all fault_in_*() (more below). v2 adds a 'min_size' argument to all fault_in_*() functions with current callers passing 0 (or we could make it 1). A probe_subpage_*() call is made for the min_size range, though with all 0 this wouldn't have any effect. The only difference is btrfs search_ioctl() in the last patch which passes a non-zero min_size to avoid the live-lock (functionally that's the same as the v1 series). In terms of sub-page probing, I don't think with the current kernel anything other than search_ioctl() matters. The buffered file I/O can already cope with current fault_in_*() + copy_*_user() loops (the uaccess makes progress). Direct I/O either goes via GUP + kernel mapping access (and memcpy() can't fault) or, if the user buffer is not PAGE aligned, it may fall back to buffered I/O. So we really only care about fault_in_writeable(), as in v1. Linus suggested that we could use the min_size to request a minimum guaranteed probed size (in most cases this would be 1) and put a cap on the faulted-in size, say two pages. All the fault_in_iov_iter_*() callers will need to check the actual quantity returned by fault_in_*() rather than bail out on non-zero but Andreas has a patch already (though I think there are a few cases in btrfs etc.): https://lore.kernel.org/r/20211123151812.361624-1-agruenba@xxxxxxxxxx With these callers fixed, we could add something like the diff below. But, again, min_size doesn't actually have any current use in the kernel other than fault_in_writeable() and search_ioctl(). Thanks for having a look. Suggestions welcomed. ------------------8<------------------------------- diff --git a/mm/gup.c b/mm/gup.c index 7fa69b0fb859..3aa88aa8ce9d 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1658,6 +1658,8 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start, } #endif /* !CONFIG_MMU */ +#define MAX_FAULT_IN_SIZE (2 * PAGE_SIZE) + /** * fault_in_writeable - fault in userspace address range for writing * @uaddr: start of address range @@ -1671,6 +1673,7 @@ size_t fault_in_writeable(char __user *uaddr, size_t size, size_t min_size) { char __user *start = uaddr, *end; size_t faulted_in = size; + size_t max_size = max_t(size_t, MAX_FAULT_IN_SIZE, min_size); if (unlikely(size == 0)) return 0; @@ -1679,7 +1682,7 @@ size_t fault_in_writeable(char __user *uaddr, size_t size, size_t min_size) return size; uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr); } - end = (char __user *)PAGE_ALIGN((unsigned long)start + size); + end = (char __user *)PAGE_ALIGN((unsigned long)start + max_size); if (unlikely(end < start)) end = NULL; while (uaddr != end) { @@ -1726,9 +1729,10 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size, struct vm_area_struct *vma = NULL; int locked = 0; size_t faulted_in = size; + size_t max_size = max_t(size_t, MAX_FAULT_IN_SIZE, min_size); nstart = start & PAGE_MASK; - end = PAGE_ALIGN(start + size); + end = PAGE_ALIGN(start + max_size); if (end < nstart) end = 0; for (; nstart != end; nstart = nend) { @@ -1759,7 +1763,7 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size, if (locked) mmap_read_unlock(mm); if (nstart != end) - faulted_in = min_t(size_t, nstart - start, size); + faulted_in = min_t(size_t, nstart - start, max_size); if (faulted_in < min_size || (min_size && probe_subpage_safe_writeable(uaddr, min_size))) return size; @@ -1782,6 +1786,7 @@ size_t fault_in_readable(const char __user *uaddr, size_t size, const char __user *start = uaddr, *end; volatile char c; size_t faulted_in = size; + size_t max_size = max_t(size_t, MAX_FAULT_IN_SIZE, min_size); if (unlikely(size == 0)) return 0; @@ -1790,7 +1795,7 @@ size_t fault_in_readable(const char __user *uaddr, size_t size, return size; uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr); } - end = (const char __user *)PAGE_ALIGN((unsigned long)start + size); + end = (const char __user *)PAGE_ALIGN((unsigned long)start + max_size); if (unlikely(end < start)) end = NULL; while (uaddr != end) { ------------------8<------------------------------- Catalin Marinas (4): mm: Introduce a 'min_size' argument to fault_in_*() mm: Probe for sub-page faults in fault_in_*() arm64: Add support for user sub-page fault probing btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults arch/Kconfig | 7 ++++ arch/arm64/Kconfig | 1 + arch/arm64/include/asm/uaccess.h | 59 +++++++++++++++++++++++++++++ arch/powerpc/kernel/kvm.c | 2 +- arch/powerpc/kernel/signal_32.c | 4 +- arch/powerpc/kernel/signal_64.c | 2 +- arch/x86/kernel/fpu/signal.c | 2 +- drivers/gpu/drm/armada/armada_gem.c | 2 +- fs/btrfs/file.c | 6 +-- fs/btrfs/ioctl.c | 7 +++- fs/f2fs/file.c | 2 +- fs/fuse/file.c | 2 +- fs/gfs2/file.c | 8 ++-- fs/iomap/buffered-io.c | 2 +- fs/ntfs/file.c | 2 +- fs/ntfs3/file.c | 2 +- include/linux/pagemap.h | 8 ++-- include/linux/uaccess.h | 53 ++++++++++++++++++++++++++ include/linux/uio.h | 6 ++- lib/iov_iter.c | 28 +++++++++++--- mm/filemap.c | 2 +- mm/gup.c | 37 +++++++++++++----- 22 files changed, 203 insertions(+), 41 deletions(-)
