The code structure in filemap_fault() has evolved significantly since these comments were added making them out of date and slightly confusing. Update them to more accurately reflect the code. Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx> --- The comments also suggest there is no performance concern in the page_not_uptodate path. This isn't entirely true, because this is a common path for MADV_RANDOM due to the lack of read-ahead/around. We always drop mmap_lock and retry when handling faults for !folio_test_uptodate(). Not dropping the lock resulted in a ~3-7% performance increase on a simple test on my local system. However we typically avoid holding mmap_lock during IO to reduce contention so not sure if changing that is worth pursuing or not? --- mm/filemap.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 9a1eef6c5d35..407420ebd6dc 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3168,8 +3168,9 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) VM_BUG_ON_FOLIO(!folio_contains(folio, index), folio); /* - * 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. + * We have a locked page in the page cache. Check that it's up-to-date. + * If not it is because there was no page in the page cache (only likely + * in the case of MADV_RANDOM) or due to error. */ if (unlikely(!folio_test_uptodate(folio))) { /* @@ -3214,10 +3215,9 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) page_not_uptodate: /* - * Umm, take care of errors if the page isn't up-to-date. - * Try to re-read it _once_. We do this synchronously, - * because there really aren't any performance issues here - * and we need to check for errors. + * Read in the page if it's not up-to-date. We have to do this + * synchronously so we can map the page to handle the fault or deal with + * errors. */ fpin = maybe_unlock_mmap_for_io(vmf, fpin); error = filemap_read_folio(file, mapping, folio); -- 2.35.1