In filemap_fault(), if don't want read-ahead, the process is as follows:
First, __filemap_get_folio alloc one new folio, because PG_uptodate is not
set, filemap_read_folio() will be called later to read data into the folio.
Secondly, before returning, it will check whether the per vma lock or
mmap_lock is released. If the lock is released, VM_FAULT_RETRY is returned,
which means that the page fault path needs retry again. Finally,
filemap_map_pages() is called in do_fault_around() to establish a page
table mapping for the previous folio.
Because filemap_read_folio() just read the data of one folio, without
read-ahead pages, it is no needs to go through the do_fault_around() again
in the page fault retry path.
Signed-off-by: Rong Qianfeng <rongqianfeng@xxxxxxxx>
---
mm/filemap.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index d62150418b91..f29adf5cf081
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3105,6 +3105,15 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
return 1;
}
+static inline bool want_readahead(unsigned long vm_flags, struct file_ra_state *ra)
+{
+ if (vm_flags & VM_RAND_READ || !ra->ra_pages)
+ return false;
+
+ return true;
+}
+
/*
* Synchronous readahead happens when we don't even find a page in the page
* cache at all. We don't want to perform IO under the mmap sem, so if we have
@@ -3141,9 +3150,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
#endif
/* If we don't want any read-ahead, don't bother */
- if (vm_flags & VM_RAND_READ)
- return fpin;
- if (!ra->ra_pages)
+ if (!want_readahead(vm_flags, ra))
return fpin;
if (vm_flags & VM_SEQ_READ) {
@@ -3191,7 +3198,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
unsigned int mmap_miss;
/* If we don't want any read-ahead, don't bother */
- if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
+ if (!want_readahead(vmf->vma->vm_flags, ra))
return fpin;
mmap_miss = READ_ONCE(ra->mmap_miss);
@@ -3612,6 +3619,14 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
unsigned long rss = 0;
unsigned int nr_pages = 0, mmap_miss = 0, mmap_miss_saved, folio_type;
+ /*
+ * If no other read-ahead pages, return zero will
+ * call __do_fault() to end the page fault path.
+ */
+ if ((vmf->flags & FAULT_FLAG_TRIED) &&
+ !want_readahead(vma->vm_flags, &file->f_ra))
+ return 0;
+
rcu_read_lock();
folio = next_uptodate_folio(&xas, mapping, end_pgoff);
if (!folio)
--
2.39.0
