From: Steven Rostedt <srostedt@xxxxxxxxxx> The function handle_mm_fault() is long overdue for comments. Adding a kernel doc header for the function and explaining the subtle use of the flags with respect to mmap_sem will prove useful in the future when others work with this code. Russell King noticed that the code in arch/x86/mm/fault.c looked buggy as the do_page_fault() code would grab the mmap_sem multiple times without letting it go. But it only did this when the handle_mm_fault() would return VM_FAULT_RETRY. Examining the code and reading the git change logs, it was discovered that commit d065bd810b6deb67d4897a14bfe21f8eb526ba99 mm: retry page fault when blocking on disk transfer added code to remove contention with the mmap_sem when the page_lock was being held for IO. As waiting on IO holding the mmap_sem can cause lots of contention between threads. The flag FAULT_FLAG_ALLOW_RETRY was added to let handle_mm_fault() know that it can safely release the mmap_sem. Adding to the confusion here with handle_mm_fault(), another commit 318b275fbca1ab9ec0862de71420e0e92c3d1aa7 mm: allow GUP to fail instead of waiting on a page was added that would not release the mmap_sem, even if FAULT_FLAG_ALLOW_RETRY was set and the page_lock was not taken and VM_FAULT_RETRY was returned, if FAULT_FLAGS_RETRY_NOWAIT was set. All of this is poorly documented and makes using or modifying handle_mm_fault() fragile. Documenting all of these subtle changes at the head of handle_mm_fault() should help future developers understand what is happening. Reported-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx> Cc: Gleb Natapov <gleb@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Michel Lespinasse <walken@xxxxxxxxxx> Cc: Avi Kivity <avi@xxxxxxxxxx> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> --- mm/memory.c | 22 ++++++++++++++++++++-- 1 files changed, 20 insertions(+), 2 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 5371b5e..3cf30f6 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3378,8 +3378,26 @@ unlock: return 0; } -/* - * By the time we get here, we already hold the mm semaphore +/** + * handle_mm_fault - main routine for handling page faults + * @mm: the mm_struct of the target address space + * @vma: vm_area_struct holding the applicable pages + * @address: the address that took the fault + * @flags: flags modifying lookup behaviour + * + * Must have @mm->mmap_sem held. + * + * Note: if @flags has FAULT_FLAG_ALLOW_RETRY set then the mmap_sem + * may be released if it failed to arquire the page_lock. If the + * mmap_sem is released then it will return VM_FAULT_RETRY set. + * This is to keep the time mmap_sem is held when the page_lock + * is taken for IO. + * Exception: If FAULT_FLAG_RETRY_NOWAIT is set, then it will + * not release the mmap_sem, but will still return VM_FAULT_RETRY + * if it failed to acquire the page_lock. + * This is for helping virtualization. See get_user_page_nowait(). + * + * Returns status flags based on the VM_FAULT_* flags in <linux/mm.h> */ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long address, unsigned int flags) -- 1.7.5.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>