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Subject[RFC v2][PATCH]page_fault retry with NOPAGE_RETRY
From
changelog[v2]:
- reduce the runtime overhead by extending the 'write' flag of
handle_mm_fault() to indicate the retry hint.
- add another two branches in filemap_fault with retry logic.
- replace find_lock_page with find_lock_page_retry to make the code
cleaner.

todo:
- there is potential a starvation hole with the retry. By the time the
retry returns, the pages might be released. we can make change by holding
page reference as well as remembering what the page "was"(in case the
file was truncated). any suggestion here are welcomed.

I also made patches for all other arch. I am posting x86_64 here first and
i will post others by the time everyone feels comfortable of this patch.

Edwin, please test this patch with your testcase and check if you get any
performance improvement of mmap over read. I added another two more places
in filemap_fault with retry logic which you might hit in your privous
experiment.


page fault retry with NOPAGE_RETRY
Allow major faults to drop the mmap_sem read lock while waiting for
synchronous disk read. This allows another thread which wishes to grab
down_write(mmap_sem) to proceed while the current is waiting the disk IO.

The patch extend the 'write' flag of handle_mm_fault() to FAULT_FLAG_RETRY
as identify that the caller can tolerate the retry in the filemap_fault call
patch.

This patch helps a lot in cases we have writer which is waitting behind all
readers, so it could execute much faster.


Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Ying Han <yinghan@google.com>


diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 31e8730..5cf5eff 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -591,6 +591,7 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigne
#ifdef CONFIG_X86_64
unsigned long flags;
#endif
+ unsigned int retry_flag = FAULT_FLAG_RETRY;

tsk = current;
mm = tsk->mm;
@@ -689,6 +690,7 @@ again:
down_read(&mm->mmap_sem);
}

+retry:
vma = find_vma(mm, address);
if (!vma)
goto bad_area;
@@ -715,6 +717,7 @@ again:
good_area:
si_code = SEGV_ACCERR;
write = 0;
+ write |= retry_flag;
switch (error_code & (PF_PROT|PF_WRITE)) {
default: /* 3: write, present */
/* fall through */
@@ -743,6 +746,15 @@ good_area:
goto do_sigbus;
BUG();
}
+
+ if (fault & VM_FAULT_RETRY) {
+ if (write & FAULT_FLAG_RETRY) {
+ retry_flag &= ~FAULT_FLAG_RETRY;
+ goto retry;
+ }
+ BUG();
+ }
+
if (fault & VM_FAULT_MAJOR)
tsk->maj_flt++;
else
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ffee2f7..9cc65a5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -144,7 +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 majoy fault */

/*
* vm_fault is filled by the the pagefault handler and passed to the vma's
@@ -694,6 +694,7 @@ static inline int page_mapped(struct page *page)
#define VM_FAULT_SIGBUS 0x0002
#define VM_FAULT_MAJOR 0x0004
#define VM_FAULT_WRITE 0x0008 /* Special case for get_user_pages */
+#define VM_FAULT_RETRY 0x0010

#define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page
#define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */
diff --git a/mm/filemap.c b/mm/filemap.c
index f3e5f89..aab4a08 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -714,6 +714,56 @@ repeat:
EXPORT_SYMBOL(find_lock_page);

/**
+ * find_lock_page_retry - locate, pin and lock a pagecache page, if retry
+ * flag is on, and page is already locked by someone else, return a hint of
+ * retry.
+ * @mapping: the address_space to search
+ * @offset: the page index
+ * @vma: vma in which the fault was taken
+ * @page: zero if page not present, otherwise point to the page in
+ * pagecache.
+ * @retry: 1 indicate caller tolerate a retry.
+ *
+ * Return *page==NULL if page is not in pagecache. Otherwise return *page
+ * 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.
+ */
+unsigned find_lock_page_retry(struct address_space *mapping, pgoff_t offset,
+ struct vm_area_struct *vma, struct page **page,
+ int retry)
+{
+ unsigned int ret = 0;
+
+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);
+ 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);
+ }
+ 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 +1494,8 @@ 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 +1510,8 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_
*/
retry_find:
page = find_lock_page(mapping, vmf->pgoff);
+
+retry_find_nopage:
/*
* For sequential accesses, we use the generic readahead logic.
*/
@@ -1465,9 +1519,12 @@ 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 (!page)
goto no_cached_page;
+ if (retry_ret == VM_FAULT_RETRY)
+ return retry_ret;
}
if (PageReadahead(page)) {
page_cache_async_readahead(mapping, ra, file, page,
@@ -1504,14 +1561,18 @@ 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_ret = find_lock_page_retry(mapping, vmf->pgoff,
+ vma, &page, retry_flag);
if (!page)
goto no_cached_page;
+ if (retry_ret == VM_FAULT_RETRY)
+ return retry_ret;
}

if (!did_readaround)
ra->mmap_miss--;

+retry_page_update:
/*
* We have a locked page in the page cache, now we need to check
* that it's up-to-date. If not, it is going to be due to an error.
@@ -1547,8 +1608,23 @@ no_cached_page:
* In the unlikely event that someone removed it in the
* meantime, we'll just come back here and read it again.
*/
- if (error >= 0)
- goto retry_find;
+ if (error >= 0) {
+ /*
+ * If caller cannot tolerate a retry in the ->fault path
+ * go back to check the page again.
+ */
+ if (!retry_flag)
+ goto retry_find;
+
+ retry_ret = find_lock_page_retry(mapping, vmf->pgoff,
+ vma, &page, retry_flag);
+ if (!page)
+ goto retry_find_nopage;
+ else if (retry_ret == VM_FAULT_RETRY)
+ return retry_ret;
+ else
+ goto retry_page_update;
+ }

/*
* An error return from page_cache_read can result if the
diff --git a/mm/memory.c b/mm/memory.c
index 164951c..1ff37f7 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
+ */
+ if (ret == VM_FAULT_RETRY)
+ return ret;
+
if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
return ret;

@@ -2613,6 +2620,7 @@ static int do_linear_fault(struct mm_struct *mm, struct
- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
unsigned int flags = (write_access ? 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);
}

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