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Message-Id: <20211201193750.2097885-1-catalin.marinas@arm.com>
Date: Wed, 1 Dec 2021 19:37:46 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
Andreas Gruenbacher <agruenba@...hat.com>
Cc: Josef Bacik <josef@...icpanda.com>,
David Sterba <dsterba@...e.com>,
Al Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Will Deacon <will@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-btrfs@...r.kernel.org
Subject: [PATCH v2 0/4] Avoid live-lock in fault-in+uaccess loops with sub-page faults
Hi,
Following the discussions on the first series,
https://lore.kernel.org/r/20211124192024.2408218-1-catalin.marinas@arm.com
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@redhat.com
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(-)
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