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Message-ID: <20250514035125.GB178093@mit.edu> Date: Tue, 13 May 2025 23:51:25 -0400 From: "Theodore Ts'o" <tytso@....edu> To: 陈涛涛 Taotao Chen <chentaotao@...iglobal.com> Cc: "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "willy@...radead.org" <willy@...radead.org>, "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org> Subject: Re: [PATCH 1/3] mm/filemap: initialize fsdata with iocb->ki_flags On Mon, Apr 21, 2025 at 10:50:30AM +0000, 陈涛涛 Taotao Chen wrote: > diff --git a/mm/filemap.c b/mm/filemap.c > index 7b90cbeb4a1a..9174b6310f0b 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4087,7 +4087,11 @@ ssize_t generic_perform_write(struct kiocb *iocb, struct iov_iter *i) > size_t offset; /* Offset into folio */ > size_t bytes; /* Bytes to write to folio */ > size_t copied; /* Bytes copied from user */ > - void *fsdata = NULL; > + /* > + * Initialize fsdata with iocb flags pointer so that filesystems > + * can check ki_flags (like IOCB_DONTCACHE) in write operations. > + */ > + void *fsdata = &iocb->ki_flags; Unfortunately, this is't safe. There may very well be code paths which depend on fsdata being initialized to NULL before calling write_begin(). In fact in the patch 2/3 in this series, ext4_write_end() will get confused in the non-delayed allocation case, since ext4_write_begin() doesn't force fsdata to be not be &iocb->ki_flags, and this will cause ext4_write_end() to potentially get confused and do the wrong thing. I understand that it would be a lot more inconvenient change the function signature of write_begin() to pass through iocb->ki_fags via a new parameter. But I think that probably is the best way to go. Cheers, - Ted
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