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Date: Sat, 23 Dec 2023 16:56:50 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>,
	ntfs3@lists.linux.dev,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 5/8] fs/ntfs3: Use kvmalloc instead of kmalloc(...
 __GFP_NOWARN)
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 <ZYZmFPnJAM3aJLlF@casper.infradead.org>
 <bca8d526-2397-4ca5-b1d6-5758c9334a81@I-love.SAKURA.ne.jp>
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On Sat, Dec 23, 2023 at 10:33:11PM +0900, Tetsuo Handa wrote:
> But you can't replace GFP_NOFS with GFP_KERNEL anyway, for syzbot is also
> reporting GFP_KERNEL allocation with filesystem lock held
> at https://syzkaller.appspot.com/bug?extid=18f543fc90dd1194c616 .

Well, you _can_.  What _all_ filesystem authors should be doing is 
switching to memalloc_nofs_save/restore.  Generally when taking a lock
that's needed during reclaim.  In this specific case, soemthing like
this:

diff --git a/fs/ntfs3/record.c b/fs/ntfs3/record.c
index 7b6423584eae..432905489a14 100644
--- a/fs/ntfs3/record.c
+++ b/fs/ntfs3/record.c
@@ -122,6 +122,7 @@ int mi_read(struct mft_inode *mi, bool is_mft)
 	struct ntfs_inode *mft_ni = sbi->mft.ni;
 	struct runs_tree *run = mft_ni ? &mft_ni->file.run : NULL;
 	struct rw_semaphore *rw_lock = NULL;
+	unsigned int memalloc = memalloc_nofs_save();
 
 	if (is_mounted(sbi)) {
 		if (!is_mft && mft_ni) {
@@ -177,6 +178,7 @@ int mi_read(struct mft_inode *mi, bool is_mft)
 		goto out;
 	}
 
+	memalloc_nofs_restore(memalloc);
 	return 0;
 
 out:
@@ -186,6 +188,7 @@ int mi_read(struct mft_inode *mi, bool is_mft)
 		err = -EINVAL;
 	}
 
+	memalloc_nofs_restore(memalloc);
 	return err;
 }