| Message ID | 20220603071233.2296100-1-apopple@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/filemap.c: Update comments in filemap_fault() | expand |
On Fri, Jun 03, 2022 at 05:12:33PM +1000, Alistair Popple wrote: > +++ 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. The comment is correct, but the code is buggy! We should be calling page_cache_ra_order() in do_sync_mmap_readahead(), even if VM_RAND_READ is set (or !ra_pages, or mmap_miss is high). At least for one page. That would allow us to drop the FGP_FOR_MMAP flag as page_cache_ra_order() will instantiate a folio for us. Do you want to write that patch or should I do it?
Matthew Wilcox <willy@infradead.org> writes: > On Fri, Jun 03, 2022 at 05:12:33PM +1000, Alistair Popple wrote: >> +++ 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. > > The comment is correct, but the code is buggy! Ha! > We should be calling page_cache_ra_order() in do_sync_mmap_readahead(), > even if VM_RAND_READ is set (or !ra_pages, or mmap_miss is high). > At least for one page. That would allow us to drop the FGP_FOR_MMAP > flag as page_cache_ra_order() will instantiate a folio for us. > > Do you want to write that patch or should I do it? Thanks for the pointers, I can do it. - Alistair
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);
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@nvidia.com> --- 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(-)