Re: [PATCH v2 1/5] netfs: clear post-EOF pagecache when extending a file via write

[Paulo Alcantara <pc@manguebit.org>]

Paulo Alcantara <pc@manguebit.org> writes:

> Fix netfs to erase the contents of a hole created after the EOF by an
> ordinary write if dirty data has been previously left there by writes
> through an mmapped region.  Neither the buffered nor the unbuffered/DIO
> write path clears that stale pagecache.
>
> Zero the tail of the folio straddling the EOF before an extending
> write.  That is the only folio that can hold data written past the EOF
> through an mmap, as pages wholly beyond the EOF can't be faulted in.
> The folio is zeroed rather than dropped so a concurrent extending write
> can't lose data.
>
> truncate_pagecache() can't be used here: it must be called with the
> i_rwsem held exclusively, but these write paths only hold it shared,
> and it would block unconditionally, breaking IOCB_NOWAIT.
>
> Export the helper so filesystems can clear the same stale data from
> their own truncate paths.
>
> Closes: https://sashiko.dev/#/patchset/20260921154957.903891-1-pc%40manguebit.org
> Fixes: 938e13a73b24 ("netfs: Implement buffered write API")
> Fixes: 153a9961b551 ("netfs: Implement unbuffered/DIO write support")
> Reviewed-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Paulo Alcantara <pc@manguebit.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Namjae Jeon <linkinjeon@kernel.org>
> Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
> Cc: Shyam Prasad N <sprasad@microsoft.com>
> Cc: Tom Talpey <tom@talpey.com>
> Cc: Bharath SM <bharathsm@microsoft.com>
> Cc: stable@vger.kernel.org
> ---
>  Documentation/filesystems/netfs_library.rst | 18 ++++++++
>  fs/netfs/buffered_write.c                   |  9 ++++
>  fs/netfs/direct_write.c                     |  9 ++++
>  fs/netfs/misc.c                             | 51 +++++++++++++++++++++
>  include/linux/netfs.h                       |  2 +
>  5 files changed, 89 insertions(+)

Will address more Sashiko comments in next version.  Please ignore this
one.