Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

[Linus Torvalds]

On Thu, Jan 17, 2019 at 4:12 AM Jiri Kosina <jikos@xxxxxxxxxx> wrote:
>
> So that seems to deal with mincore() in a reasonable way indeed.
>
> It doesn't unfortunately really solve the preadv2(RWF_NOWAIT), nor does it
> provide any good answer what to do about it, does it?

As I suggested earlier in the thread, the fix for RWF_NOWAIT might be
to just move the test down to after readahead.

We could/should be smarter than this,but it *might* be as simple as just

diff --git a/mm/filemap.c b/mm/filemap.c
index 9f5e323e883e..7bcdd36e629d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2075,8 +2075,6 @@ static ssize_t generic_file_buffered_read(

                page = find_get_page(mapping, index);
                if (!page) {
-                       if (iocb->ki_flags & IOCB_NOWAIT)
-                               goto would_block;
                        page_cache_sync_readahead(mapping,
                                        ra, filp,
                                        index, last_index - index);

which starts readahead even if IOCB_NOWAIT is set and will then test
IOCB_NOWAIT _later_ and not actually wait for it.

            Linus