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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 17 Jan 2019 05:48:33 +1200
Message-ID: <CAHk-=wgsnWvSsMfoEYzOq6fpahkHWxF3aSJBbVqywLa34OXnLg@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Jiri Kosina <jikos@kernel.org>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Josh Snyder <joshs@netflix.com>,
        Dave Chinner <david@fromorbit.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Thu, Jan 17, 2019 at 4:12 AM Jiri Kosina <jikos@kernel.org> 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