Messages in this thread | | | Date | Thu, 9 Apr 2009 23:32:01 -0700 | Subject | Re: [PATCH][1/2]page_fault retry with NOPAGE_RETRY | From | Ying Han <> |
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2009/4/9 Andrew Morton <akpm@linux-foundation.org>: > >> Subject: [PATCH][1/2]page_fault retry with NOPAGE_RETRY > > Please give each patch in the series a unique and meaningful title. > > On Wed, 8 Apr 2009 13:02:35 -0700 Ying Han <yinghan@google.com> wrote: > >> support for FAULT_FLAG_RETRY with no user change: > > yup, we'd prefer a complete changelog here please. > >> Signed-off-by: Ying Han <yinghan@google.com> >> Mike Waychison <mikew@google.com> > > This form: > > Signed-off-by: Ying Han <yinghan@google.com> > Signed-off-by: Mike Waychison <mikew@google.com>
Thanks Andrew, and i need to add Fengguang to Signed-off-by.
> > is conventional. > >> index 4a853ef..29c2c39 100644 >> --- a/include/linux/fs.h >> +++ b/include/linux/fs.h >> @@ -793,7 +793,7 @@ struct file_ra_state { >> there are only # of pages ahead */ >> >> unsigned int ra_pages; /* Maximum readahead window */ >> - int mmap_miss; /* Cache miss stat for mmap accesses */ >> + unsigned int mmap_miss; /* Cache miss stat for mmap accesses */ > > This change makes sense, but we're not told the reasons for making it? > Did it fix a bug, or is it an unrelated fixlet, or...?
Fengguang: Could you help making comments on this part? and i will make changes elsewhere as Andrew pointed. Thanks
> >> loff_t prev_pos; /* Cache last read() position */ >> }; >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index ffee2f7..5a134a9 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -144,6 +144,7 @@ extern pgprot_t protection_map[16]; >> >> #define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */ >> #define FAULT_FLAG_NONLINEAR 0x02 /* Fault was via a nonlinear mapping */ >> +#define FAULT_FLAG_RETRY 0x04 /* Retry major fault */ >> >> >> /* >> @@ -690,6 +691,7 @@ static inline int page_mapped(struct page *page) >> >> #define VM_FAULT_MINOR 0 /* For backwards compat. Remove me quickly. */ >> >> +#define VM_FAULT_RETRY 0x0010 >> #define VM_FAULT_OOM 0x0001 >> #define VM_FAULT_SIGBUS 0x0002 >> #define VM_FAULT_MAJOR 0x0004 >> diff --git a/mm/filemap.c b/mm/filemap.c >> index f3e5f89..6eb7c36 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -714,6 +714,58 @@ repeat: >> EXPORT_SYMBOL(find_lock_page); >> >> /** >> + * find_lock_page_retry - locate, pin and lock a pagecache page >> + * @mapping: the address_space to search >> + * @offset: the page index >> + * @vma: vma in which the fault was taken >> + * @ppage: zero if page not present, otherwise point to the page in pagecache >> + * @retry: 1 indicate caller tolerate a retry. >> + * >> + * If retry flag is on, and page is already locked by someone else, return >> + * a hint of retry and leave *ppage untouched. >> + * >> + * Return *ppage==NULL if page is not in pagecache. Otherwise return *ppage >> + * points to the page in the pagecache with ret=VM_FAULT_RETRY indicate a >> + * hint to caller for retry, or ret=0 which means page is succefully >> + * locked. >> + */ > > How about this: > > If the page was not found in pagecache, find_lock_page_retry() > returns 0 and sets *@ppage to NULL. > > If the page was found in pagecache but is locked and @retry is > true, find_lock_page_retry() returns VM_FAULT_RETRY and does not > write to *@ppage. > > If the page was found in pagecache and @retry is false, > find_lock_page_retry() locks the page, writes its address to *@ppage > and returns 0. > >> +unsigned find_lock_page_retry(struct address_space *mapping, pgoff_t offset, >> + struct vm_area_struct *vma, struct page **ppage, >> + int retry) >> +{ >> + unsigned int ret = 0; >> + struct page *page; >> + >> +repeat: >> + page = find_get_page(mapping, offset); >> + if (page) { >> + if (!retry) >> + lock_page(page); >> + else { >> + if (!trylock_page(page)) { >> + struct mm_struct *mm = vma->vm_mm; >> + >> + up_read(&mm->mmap_sem); >> + wait_on_page_locked(page); > > It looks strange that we wait for the page lock and then don't > just lock the page and return it. > > Adding a comment explaining _why_ we do this would be nice. > >> + down_read(&mm->mmap_sem); >> + >> + page_cache_release(page); >> + return VM_FAULT_RETRY; >> + } >> + } >> + if (unlikely(page->mapping != mapping)) { >> + unlock_page(page); >> + page_cache_release(page); >> + goto repeat; >> + } >> + VM_BUG_ON(page->index != offset); >> + } >> + *ppage = page; >> + return ret; >> +} >> +EXPORT_SYMBOL(find_lock_page_retry); >> + >> +/** >> * find_or_create_page - locate or add a pagecache page >> * @mapping: the page's address_space >> * @index: the page's index into the mapping >> @@ -1444,6 +1496,8 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_ >> pgoff_t size; >> int did_readaround = 0; >> int ret = 0; >> + int retry_flag = vmf->flags & FAULT_FLAG_RETRY; >> + int retry_ret; >> >> size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT; >> if (vmf->pgoff >= size) >> @@ -1458,6 +1512,7 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_ >> */ >> retry_find: >> page = find_lock_page(mapping, vmf->pgoff); >> + >> /* >> * For sequential accesses, we use the generic readahead logic. >> */ >> @@ -1465,7 +1520,13 @@ retry_find: >> if (!page) { >> page_cache_sync_readahead(mapping, ra, file, >> vmf->pgoff, 1); >> - page = find_lock_page(mapping, vmf->pgoff); >> + retry_ret = find_lock_page_retry(mapping, vmf->pgoff, >> + vma, &page, retry_flag); >> + if (retry_ret == VM_FAULT_RETRY) { >> + /* counteract the followed retry hit */ >> + ra->mmap_miss++; >> + return retry_ret; >> + } >> if (!page) >> goto no_cached_page; >> } >> @@ -1504,7 +1565,14 @@ retry_find: >> start = vmf->pgoff - ra_pages / 2; >> do_page_cache_readahead(mapping, file, start, ra_pages); >> } >> - page = find_lock_page(mapping, vmf->pgoff); >> +retry_find_retry: >> + retry_ret = find_lock_page_retry(mapping, vmf->pgoff, >> + vma, &page, retry_flag); >> + if (retry_ret == VM_FAULT_RETRY) { >> + /* counteract the followed retry hit */ >> + ra->mmap_miss++; >> + return retry_ret; >> + } >> if (!page) >> goto no_cached_page; >> } >> @@ -1548,7 +1616,7 @@ no_cached_page: >> * meantime, we'll just come back here and read it again. >> */ >> if (error >= 0) >> - goto retry_find; >> + goto retry_find_retry; >> >> /* >> * An error return from page_cache_read can result if the >> diff --git a/mm/memory.c b/mm/memory.c >> index 164951c..5e215c9 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2467,6 +2467,13 @@ static int __do_fault(struct mm_struct *mm, struct vm_a >> vmf.page = NULL; >> >> ret = vma->vm_ops->fault(vma, &vmf); >> + >> + /* page may be available, but we have to restart the process >> + * because mmap_sem was dropped during the ->fault >> + */ > > The preferred comment layout is: > > /* > * The page may be available, but we have to restart the process > * because mmap_sem was dropped during the ->fault > */ > >> + if (ret & VM_FAULT_RETRY) >> + return ret; > > Again, I worry that in some places we treat VM_FAULT_RETRY as an > enumerated type: > > if (foo == VM_FAULT_RETRY) > > but in other places we treat it as a bitfield > > if (foo & VM_FAULT_RETRY) > > it may not be buggy, but it _looks_ buggy. And confusing and > inconsistent and dangerous. > > Can it be improved? > >> if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))) >> return ret; >> >> @@ -2611,8 +2618,10 @@ static int do_linear_fault(struct mm_struct *mm, struct >> { >> pgoff_t pgoff = (((address & PAGE_MASK) >> - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff; >> - unsigned int flags = (write_access ? FAULT_FLAG_WRITE : 0); >> + int write = write_access & ~FAULT_FLAG_RETRY; >> + unsigned int flags = (write ? FAULT_FLAG_WRITE : 0); >> >> + flags |= (write_access & FAULT_FLAG_RETRY); > > gee, I'm lost. > > Can we please redo this as: > > > int write; > unsigned int flags; > > /* > * Big fat comment explaining the next three lines goes here > */ > write = write_access & ~FAULT_FLAG_RETRY; > unsigned int flags = (write ? FAULT_FLAG_WRITE : 0); > flags |= (write_access & FAULT_FLAG_RETRY); > > ? > >> pte_unmap(page_table); >> return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte); >> } >> @@ -2726,26 +2735,32 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_ar >> pud_t *pud; >> pmd_t *pmd; >> pte_t *pte; >> + int ret; >> >> __set_current_state(TASK_RUNNING); >> >> - count_vm_event(PGFAULT); >> - >> - if (unlikely(is_vm_hugetlb_page(vma))) >> - return hugetlb_fault(mm, vma, address, write_access); >> + if (unlikely(is_vm_hugetlb_page(vma))) { >> + ret = hugetlb_fault(mm, vma, address, write_access); >> + goto out; >> + } >> >> + ret = VM_FAULT_OOM; >> pgd = pgd_offset(mm, address); >> pud = pud_alloc(mm, pgd, address); >> if (!pud) >> - return VM_FAULT_OOM; >> + goto out; >> pmd = pmd_alloc(mm, pud, address); >> if (!pmd) >> - return VM_FAULT_OOM; >> + goto out; >> pte = pte_alloc_map(mm, pmd, address); >> if (!pte) >> - return VM_FAULT_OOM; >> + goto out; >> >> - return handle_pte_fault(mm, vma, address, pte, pmd, write_access); >> + ret = handle_pte_fault(mm, vma, address, pte, pmd, write_access); >> +out: >> + if (!(ret & VM_FAULT_RETRY)) >> + count_vm_event(PGFAULT); >> + return ret; >> } > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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