From 9a95f3cf7b33d66fa64727cff8cd2f2a9d09f335 Mon Sep 17 00:00:00 2001 From: Paul Cassella Date: Wed, 6 Aug 2014 16:07:24 -0700 Subject: mm: describe mmap_sem rules for __lock_page_or_retry() and callers Add a comment describing the circumstances in which __lock_page_or_retry() will or will not release the mmap_sem when returning 0. Add comments to lock_page_or_retry()'s callers (filemap_fault(), do_swap_page()) noting the impact on VM_FAULT_RETRY returns. Add comments on up the call tree, particularly replacing the false "We return with mmap_sem still held" comments. Signed-off-by: Paul Cassella Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/filemap.c | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+) (limited to 'mm/filemap.c') diff --git a/mm/filemap.c b/mm/filemap.c index 7e85c8147e1b..af19a6b079f5 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -808,6 +808,17 @@ int __lock_page_killable(struct page *page) } EXPORT_SYMBOL_GPL(__lock_page_killable); +/* + * Return values: + * 1 - page is locked; mmap_sem is still held. + * 0 - page is not locked. + * mmap_sem has been released (up_read()), unless flags had both + * FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT set, in + * which case mmap_sem is still held. + * + * If neither ALLOW_RETRY nor KILLABLE are set, will always return 1 + * with the page locked and the mmap_sem unperturbed. + */ int __lock_page_or_retry(struct page *page, struct mm_struct *mm, unsigned int flags) { @@ -1827,6 +1838,18 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma, * The goto's are kind of ugly, but this streamlines the normal case of having * it in the page cache, and handles the special cases reasonably without * having a lot of duplicated code. + * + * vma->vm_mm->mmap_sem must be held on entry. + * + * If our return value has VM_FAULT_RETRY set, it's because + * lock_page_or_retry() returned 0. + * The mmap_sem has usually been released in this case. + * See __lock_page_or_retry() for the exception. + * + * If our return value does not have VM_FAULT_RETRY set, the mmap_sem + * has not been released. + * + * We never return with VM_FAULT_RETRY and a bit from VM_FAULT_ERROR set. */ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) { -- cgit v1.2.2