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Subject[PATCH 2/7] mm: drop mmap_sem for page cache read IO submission
Date
From: Johannes Weiner <hannes@cmpxchg.org>

Reads can take a long time, and if anybody needs to take a write lock on
the mmap_sem it'll block any subsequent readers to the mmap_sem while
the read is outstanding, which could cause long delays. Instead drop
the mmap_sem if we do any reads at all.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
mm/filemap.c | 119 ++++++++++++++++++++++++++++++++++++++++++++---------------
1 file changed, 90 insertions(+), 29 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 52517f28e6f4..1ed35cd99b2c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2366,6 +2366,18 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
EXPORT_SYMBOL(generic_file_read_iter);

#ifdef CONFIG_MMU
+static struct file *maybe_unlock_mmap_for_io(struct vm_area_struct *vma, int flags)
+{
+ if ((flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT)) == FAULT_FLAG_ALLOW_RETRY) {
+ struct file *file;
+
+ file = get_file(vma->vm_file);
+ up_read(&vma->vm_mm->mmap_sem);
+ return file;
+ }
+ return NULL;
+}
+
/**
* page_cache_read - adds requested page to the page cache if not already there
* @file: file to read
@@ -2405,23 +2417,28 @@ static int page_cache_read(struct file *file, pgoff_t offset, gfp_t gfp_mask)
* Synchronous readahead happens when we don't even find
* a page in the page cache at all.
*/
-static void do_sync_mmap_readahead(struct vm_area_struct *vma,
- struct file_ra_state *ra,
- struct file *file,
- pgoff_t offset)
+static int do_sync_mmap_readahead(struct vm_area_struct *vma,
+ struct file_ra_state *ra,
+ struct file *file,
+ pgoff_t offset,
+ int flags)
{
struct address_space *mapping = file->f_mapping;
+ struct file *fpin;

/* If we don't want any read-ahead, don't bother */
if (vma->vm_flags & VM_RAND_READ)
- return;
+ return 0;
if (!ra->ra_pages)
- return;
+ return 0;

if (vma->vm_flags & VM_SEQ_READ) {
+ fpin = maybe_unlock_mmap_for_io(vma, flags);
page_cache_sync_readahead(mapping, ra, file, offset,
ra->ra_pages);
- return;
+ if (fpin)
+ fput(fpin);
+ return fpin ? -EAGAIN : 0;
}

/* Avoid banging the cache line if not needed */
@@ -2433,7 +2450,9 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
* stop bothering with read-ahead. It will only hurt.
*/
if (ra->mmap_miss > MMAP_LOTSAMISS)
- return;
+ return 0;
+
+ fpin = maybe_unlock_mmap_for_io(vma, flags);

/*
* mmap read-around
@@ -2442,28 +2461,40 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
ra->size = ra->ra_pages;
ra->async_size = ra->ra_pages / 4;
ra_submit(ra, mapping, file);
+
+ if (fpin)
+ fput(fpin);
+
+ return fpin ? -EAGAIN : 0;
}

/*
* Asynchronous readahead happens when we find the page and PG_readahead,
* so we want to possibly extend the readahead further..
*/
-static void do_async_mmap_readahead(struct vm_area_struct *vma,
- struct file_ra_state *ra,
- struct file *file,
- struct page *page,
- pgoff_t offset)
+static int do_async_mmap_readahead(struct vm_area_struct *vma,
+ struct file_ra_state *ra,
+ struct file *file,
+ struct page *page,
+ pgoff_t offset,
+ int flags)
{
struct address_space *mapping = file->f_mapping;
+ struct file *fpin;

/* If we don't want any read-ahead, don't bother */
if (vma->vm_flags & VM_RAND_READ)
- return;
+ return 0;
if (ra->mmap_miss > 0)
ra->mmap_miss--;
- if (PageReadahead(page))
- page_cache_async_readahead(mapping, ra, file,
- page, offset, ra->ra_pages);
+ if (!PageReadahead(page))
+ return 0;
+ fpin = maybe_unlock_mmap_for_io(vma, flags);
+ page_cache_async_readahead(mapping, ra, file,
+ page, offset, ra->ra_pages);
+ if (fpin)
+ fput(fpin);
+ return fpin ? -EAGAIN : 0;
}

/**
@@ -2479,10 +2510,8 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma,
*
* 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 has VM_FAULT_RETRY set, the mmap_sem has
+ * usually been released.
*
* If our return value does not have VM_FAULT_RETRY set, the mmap_sem
* has not been released.
@@ -2492,11 +2521,13 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma,
vm_fault_t filemap_fault(struct vm_fault *vmf)
{
int error;
+ struct mm_struct *mm = vmf->vma->vm_mm;
struct file *file = vmf->vma->vm_file;
struct address_space *mapping = file->f_mapping;
struct file_ra_state *ra = &file->f_ra;
struct inode *inode = mapping->host;
pgoff_t offset = vmf->pgoff;
+ int flags = vmf->flags;
pgoff_t max_off;
struct page *page;
vm_fault_t ret = 0;
@@ -2509,27 +2540,44 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
* Do we have something in the page cache already?
*/
page = find_get_page(mapping, offset);
- if (likely(page) && !(vmf->flags & FAULT_FLAG_TRIED)) {
+ if (likely(page) && !(flags & FAULT_FLAG_TRIED)) {
/*
* We found the page, so try async readahead before
* waiting for the lock.
*/
- do_async_mmap_readahead(vmf->vma, ra, file, page, offset);
+ error = do_async_mmap_readahead(vmf->vma, ra, file, page, offset, vmf->flags);
+ if (error == -EAGAIN)
+ goto out_retry_wait;
} else if (!page) {
/* No page in the page cache at all */
- do_sync_mmap_readahead(vmf->vma, ra, file, offset);
- count_vm_event(PGMAJFAULT);
- count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
ret = VM_FAULT_MAJOR;
+ count_vm_event(PGMAJFAULT);
+ count_memcg_event_mm(mm, PGMAJFAULT);
+ error = do_sync_mmap_readahead(vmf->vma, ra, file, offset, vmf->flags);
+ if (error == -EAGAIN)
+ goto out_retry_wait;
retry_find:
page = find_get_page(mapping, offset);
if (!page)
goto no_cached_page;
}

- if (!lock_page_or_retry(page, vmf->vma->vm_mm, vmf->flags)) {
- put_page(page);
- return ret | VM_FAULT_RETRY;
+ if (!trylock_page(page)) {
+ if (flags & FAULT_FLAG_ALLOW_RETRY) {
+ if (flags & FAULT_FLAG_RETRY_NOWAIT)
+ goto out_retry;
+ up_read(&mm->mmap_sem);
+ goto out_retry_wait;
+ }
+ if (flags & FAULT_FLAG_KILLABLE) {
+ int ret = __lock_page_killable(page);
+
+ if (ret) {
+ up_read(&mm->mmap_sem);
+ goto out_retry;
+ }
+ } else
+ __lock_page(page);
}

/* Did it get truncated? */
@@ -2607,6 +2655,19 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
/* Things didn't work out. Return zero to tell the mm layer so. */
shrink_readahead_size_eio(file, ra);
return VM_FAULT_SIGBUS;
+
+out_retry_wait:
+ if (page) {
+ if (flags & FAULT_FLAG_KILLABLE)
+ wait_on_page_locked_killable(page);
+ else
+ wait_on_page_locked(page);
+ }
+
+out_retry:
+ if (page)
+ put_page(page);
+ return ret | VM_FAULT_RETRY;
}
EXPORT_SYMBOL(filemap_fault);

--
2.14.3
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