Re: [RFC PATCH 1/1] iomap: fix race between iomap_set_range_uptodate and folio_end_read

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 24, 2025 at 10:43:58AM -0500, Sasha Levin wrote:
> On Wed, Dec 24, 2025 at 02:10:19AM +0000, Matthew Wilcox wrote:
> > So Sasha has produced a very convincingly worded writeup that's
> > hallucinated.
> 
> And spent a few hours trying to figure it out so I could unblock testing, but
> sure - thanks.

When you produce a convincingly worded writeup that's utterly wrong,
and have a reputation for using AI, that's the kind of reaction you're
going to get.

> Here's the full log:
> https://qa-reports.linaro.org/lkft/sashal-linus-next/build/v6.18-rc7-13806-gb927546677c8/testrun/30618654/suite/log-parser-test/test/exception-warning-fsiomapbuffered-io-at-ifs_free/log
> , happy to test any patches you might have.

That's actually much more helpful because it removes your incorrect
assumptions about what's going on.

 WARNING: fs/iomap/buffered-io.c:254 at ifs_free+0x130/0x148, CPU#0: msync04/406

That's this one:

        WARN_ON_ONCE(ifs_is_fully_uptodate(folio, ifs) !=
                        folio_test_uptodate(folio));

which would be fully explained by fuse calling folio_clear_uptodate()
in fuse_send_write_pages().  I have come to believe that allowing
filesystems to call folio_clear_uptodate() is just dangerous.  It
causes assertions to fire all over the place (eg if the page is mapped
into memory, the MM contains assertions that it must be uptodate).

So I think the first step is simply to delete the folio_clear_uptodate()
calls in fuse:

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 01bc894e9c2b..b819ede407d5 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1194,7 +1194,6 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
 	struct fuse_file *ff = file->private_data;
 	struct fuse_mount *fm = ff->fm;
 	unsigned int offset, i;
-	bool short_write;
 	int err;
 
 	for (i = 0; i < ap->num_folios; i++)
@@ -1209,22 +1208,16 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
 	if (!err && ia->write.out.size > count)
 		err = -EIO;
 
-	short_write = ia->write.out.size < count;
 	offset = ap->descs[0].offset;
 	count = ia->write.out.size;
 	for (i = 0; i < ap->num_folios; i++) {
 		struct folio *folio = ap->folios[i];
 
-		if (err) {
-			folio_clear_uptodate(folio);
-		} else {
+		if (!err) {
 			if (count >= folio_size(folio) - offset)
 				count -= folio_size(folio) - offset;
-			else {
-				if (short_write)
-					folio_clear_uptodate(folio);
+			else
 				count = 0;
-			}
 			offset = 0;
 		}
 		if (ia->write.folio_locked && (i == ap->num_folios - 1))