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Date: Thu, 9 Apr 2009 23:02:05 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ying Han <yinghan@...gle.com>
Cc: linux-mm@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
torvalds@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
Mike Waychison <mikew@...gle.com>,
Rohit Seth <rohitseth@...gle.com>,
Hugh Dickins <hugh@...itas.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
"H. Peter Anvin" <hpa@...or.com>,
Török Edwin <edwintorok@...il.com>,
Lee Schermerhorn <lee.schermerhorn@...com>,
Nick Piggin <npiggin@...e.de>,
Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH][1/2]page_fault retry with NOPAGE_RETRY
> 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@...gle.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@...gle.com>
> Mike Waychison <mikew@...gle.com>
This form:
Signed-off-by: Ying Han <yinghan@...gle.com>
Signed-off-by: Mike Waychison <mikew@...gle.com>
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...?
> 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 *@...ge 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 *@...ge.
If the page was found in pagecache and @retry is false,
find_lock_page_retry() locks the page, writes its address to *@...ge
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;
> }
--
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