The patch titled Subject: mm: introduce fault_in_exact_writeable() to probe for sub-page faults has been removed from the -mm tree. Its filename was mm-introduce-fault_in_exact_writeable-to-probe-for-sub-page-faults.patch
This patch was dropped because an updated version will be merged
------------------------------------------------------ From: Catalin Marinas catalin.marinas@arm.com Subject: mm: introduce fault_in_exact_writeable() to probe for sub-page faults
Patch series "Avoid live-lock in fault-in+uaccess loops with sub-page faults".
There are a few places in the filesystem layer where a uaccess is performed in a loop with page faults disabled, together with a fault_in_*() call to pre-fault the pages. On architectures like arm64 with MTE (memory tagging extensions) or SPARC ADI, even if the fault_in_*() succeeded, the uaccess can still fault indefinitely.
In general this is not an issue since such code restarts the fault_in_*() from where the uaccess failed, therefore guaranteeing forward progress. The btrfs search_ioctl(), however, rewinds the fault_in_*() position and it can live-lock. This was reported by Al here:
https://lore.kernel.org/r/YSqOUb7yZ7kBoKRY@zeniv-ca.linux.org.uk
There's also an analysis by Al of other fault-in places:
https://lore.kernel.org/r/YSldx9uhMYhT/G8X@zeniv-ca.linux.org.uk
and another sub-thread on the same topic:
https://lore.kernel.org/r/YXBFqD9WVuU8awIv@arm.com
So far only btrfs search_ioctl() seems to be affected and that's what this series addresses. The existing loops like generic_perform_write() already guarantee forward progress.
Andreas raised a concern about O_DIRECT accesses since on fault the user address is rewound to a block size boundary. I tried ext4, btrfs and gfs2 and I could not get any of them to live-lock. Depending on the alignment of the user buffer (page or not), I found two behaviours:
- the copy to or from the user buffer succeeds entirely if it goes through the kernel mapping (GUP, kmap'ed page; user MTE tags are not checked) or
- the copy partially succeeds after a few attempts at uaccess on the faulting same address (the highest number of attempts in my tests was 11 with btrfs).
Given the high cost of such sub-page probing (which is done prior to the uaccess) my proposal is to only change the btrfs search_ioctl() (as per the last patch). We can extend the API and call places in the future if needed but I hope filesystems already deal with this in other ways.
This patch (of 3):
On hardware with features like arm64 MTE or SPARC ADI, an access fault can be triggered at sub-page granularity. Depending on how the fault_in_*() functions are used, the caller can get into a live-lock by continuously retrying the fault-in on an address different from the one where the uaccess failed.
In the majority of cases progress is ensured by the following conditions:
1. copy_{to,from}_user() guarantees at least one byte access if the user address is not faulting;
2. The fault_in_*() is attempted on the next address that could not be accessed by copy_*_user().
In the places where the above conditions are not met or the fault-in/uaccess loop does not have a mechanism to bail out, the fault_in_exact_writeable() ensures that the arch code will probe the range in question at a sub-page fault granularity (e.g. 16 bytes for arm64 MTE). For large ranges, this is significantly more expensive than the non-exact versions which probe a single byte in each page or use GUP.
The architecture code has to select ARCH_HAS_SUBPAGE_FAULTS and implement probe_user_writeable().
Link: https://lkml.kernel.org/r/20211124192024.2408218-1-catalin.marinas@arm.com Link: https://lkml.kernel.org/r/20211124192024.2408218-2-catalin.marinas@arm.com Fixes: a48b73eca4ce ("btrfs: fix potential deadlock in the search ioctl") Signed-off-by: Catalin Marinas catalin.marinas@arm.com Cc: Matthew Wilcox willy@infradead.org Cc: Al Viro viro@zeniv.linux.org.uk Cc: Andreas Gruenbacher agruenba@redhat.com Cc: David Sterba dsterba@suse.com Cc: Josef Bacik josef@toxicpanda.com Cc: Will Deacon will@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
arch/Kconfig | 7 +++++++ include/linux/pagemap.h | 1 + include/linux/uaccess.h | 21 +++++++++++++++++++++ mm/gup.c | 19 +++++++++++++++++++ 4 files changed, 48 insertions(+)
--- a/arch/Kconfig~mm-introduce-fault_in_exact_writeable-to-probe-for-sub-page-faults +++ a/arch/Kconfig @@ -27,6 +27,13 @@ config HAVE_IMA_KEXEC config SET_FS bool
+config ARCH_HAS_SUBPAGE_FAULTS + bool + help + Select if the architecture can check permissions at sub-page + granularity (e.g. arm64 MTE). The probe_user_*() functions + must be implemented. + config HOTPLUG_SMT bool
--- a/include/linux/pagemap.h~mm-introduce-fault_in_exact_writeable-to-probe-for-sub-page-faults +++ a/include/linux/pagemap.h @@ -925,6 +925,7 @@ void folio_add_wait_queue(struct folio * * Fault in userspace address range. */ size_t fault_in_writeable(char __user *uaddr, size_t size); +size_t fault_in_exact_writeable(char __user *uaddr, size_t size); size_t fault_in_safe_writeable(const char __user *uaddr, size_t size); size_t fault_in_readable(const char __user *uaddr, size_t size);
--- a/include/linux/uaccess.h~mm-introduce-fault_in_exact_writeable-to-probe-for-sub-page-faults +++ a/include/linux/uaccess.h @@ -271,6 +271,27 @@ static inline bool pagefault_disabled(vo */ #define faulthandler_disabled() (pagefault_disabled() || in_atomic())
+#ifndef CONFIG_ARCH_HAS_SUBPAGE_FAULTS +/** + * probe_user_writable: probe for sub-page faults in the user range + * @uaddr: start of address range + * @size: size of address range + * + * Returns the number of bytes not accessible (like copy_to_user() and + * copy_from_user()). + * + * Architectures that can generate sub-page faults (e.g. arm64 MTE) should + * implement this function. It is expected that the caller checked for the + * write permission of each page in the range either by put_user() or GUP. + * The architecture port can implement a more efficient get_user() probing of + * the range if sub-page faults are triggered by either a load or store. + */ +static inline size_t probe_user_writable(void __user *uaddr, size_t size) +{ + return 0; +} +#endif + #ifndef ARCH_HAS_NOCACHE_UACCESS
static inline __must_check unsigned long --- a/mm/gup.c~mm-introduce-fault_in_exact_writeable-to-probe-for-sub-page-faults +++ a/mm/gup.c @@ -1699,6 +1699,25 @@ out: } EXPORT_SYMBOL(fault_in_writeable);
+/** + * fault_in_exact_writeable - fault in userspace address range for writing, + * potentially checking for sub-page faults + * @uaddr: start of address range + * @size: size of address range + * + * Returns the number of bytes not faulted in (like copy_to_user() and + * copy_from_user()). + */ +size_t fault_in_exact_writeable(char __user *uaddr, size_t size) +{ + size_t accessible = size - fault_in_writeable(uaddr, size); + + if (accessible) + accessible -= probe_user_writable(uaddr, accessible); + return size - accessible; +} +EXPORT_SYMBOL(fault_in_exact_writeable); + /* * fault_in_safe_writeable - fault in an address range for writing * @uaddr: start of address range _
Patches currently in -mm which might be from catalin.marinas@arm.com are
arm64-add-support-for-sub-page-faults-user-probing.patch btrfs-avoid-live-lock-in-search_ioctl-on-hardware-with-sub-page-faults.patch