linux-kernel - Re: [PATCH v11 1/7] userfaultfd: Add UFFD WP Async support

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Date:   Tue, 21 Mar 2023 15:25:26 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v11 1/7] userfaultfd: Add UFFD WP Async support

Hi, Muhammad,

On Tue, Mar 21, 2023 at 05:21:15PM +0500, Muhammad Usama Anjum wrote:
> Thank you so much for the patch. I've tested hugetlb mem. This patch is
> working fine for hugetlb shmem:
> *shmid = shmget(2, size, SHM_HUGETLB | IPC_CREAT | SHM_R | SHM_W);
> mem = shmat(*shmid, 0, 0);
> 
> I've found slight issue with hugetlb mem which has been mmaped:
> mem = mmap(NULL, size, PROT_READ | PROT_WRITE,
> 	   MAP_ANONYMOUS | MAP_HUGETLB | MAP_PRIVATE, -1, 0);
> The issue is that even after witting to this memory, the wp flag is still
> present there and memory doesn't appear to be dirty when it should have
> been dirty. The temporary fix is to write to memory and write protect the
> memory one extra time.

I looked into this today and found it's an existing bug that can trigger
with sync mode too.. as long as protection applied to unpopulated hugetlb
private mappings, then write to it.

I've sent a fix for it here and have you copied:

https://lore.kernel.org/linux-mm/20230321191840.1897940-1-peterx@redhat.com/T/#u

Please have a look and see whether it also fixes your issue.

PS: recently I added a warning in commit c2da319c2e2789 and that can indeed
capture this one when verifying using pagemap.  I'd guess your dmesg should
also contain something dumped.

Thanks,

-- 
Peter Xu