block: enable RWF_DONTCACHE for block devices - #1040
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Move the atomic context detection logic from erofs's z_erofs_in_atomic() into the block layer as bio_in_atomic(). This helper returns true when the current context is unsafe for sleeping bio completion handlers (e.g., hard/soft IRQ, preempt-disabled). The logic was originally added to erofs in commit c99fab6 ("erofs: fix atomic context detection when !CONFIG_DEBUG_LOCK_ALLOC"). A subsequent patch will use it in the block layer's bio completion infrastructure, so move it to include/linux/bio.h where both subsystems can share it. Convert erofs to call the new bio_in_atomic() directly. Suggested-by: Christoph Hellwig <hch@infradead.org> Signed-off-by: Tal Zussman <tz2294@columbia.edu> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Jan Kara <jack@suse.cz>
Some bio completion handlers need to run from preemptible task context,
but bio_endio() may be called from IRQ context (e.g., buffer_head
writeback). Callers need a way to ensure their callback eventually runs
from a sleepable context. Add infrastructure for that, in two forms:
1. BIO_COMPLETE_IN_TASK, a bio flag the submitter sets when it knows
in advance that its callback needs task context (e.g., dropbehind
writeback). bio_endio() sees the flag and offloads completion to a
worker automatically.
2. bio_complete_in_task(), a helper that completion callbacks can
invoke from within bi_end_io() when the deferral decision is
dynamic (e.g., fserror reporting).
Both share a per-CPU list drained by a work item on a WQ_PERCPU
workqueue. Producers push the bio onto the local CPU's list and schedule
the work item, which then dispatches each bio's bi_end_io() from task
context.
Both methods are gated on bio_in_atomic(), which returns true in any
context where a sleeping bi_end_io() is unsafe, including
non-preemptible task context.
Two CPU hotplug callbacks are used to drain remaining bios from the
departing CPU's batch, while maintaining the per-CPU behavior. The
CPUHP_AP_ONLINE_DYN callback disables the per-CPU work item while the
CPU is still online, preventing it from running on an unbound worker
later. CPUHP_BP_PREPARE_DYN then drains any bios added between disabling
the work item and CPU offline.
Link: https://lore.kernel.org/all/20260409160243.1008358-1-hch@lst.de/
Suggested-by: Matthew Wilcox <willy@infradead.org>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Set BIO_COMPLETE_IN_TASK on iomap writeback bios when a dropbehind folio is added. This ensures that bi_end_io runs in task context, where folio_end_dropbehind() can safely invalidate folios. With the bio layer now handling task-context deferral generically, IOMAP_IOEND_DONTCACHE is no longer needed, as XFS no longer needs to route DONTCACHE ioends through its completion workqueue. Remove the flag and its NOMERGE entry. Without the NOMERGE, regular I/Os that get merged with a dropbehind folio will also have their completion deferred to task context. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Set BIO_COMPLETE_IN_TASK in __bh_submit() for write bios when the folio has dropbehind set, so that buffer_head writeback completions get deferred to task context where folio_end_dropbehind() can safely invalidate folios. Read completions are not deferred since dropbehind invalidation for reads is handled synchronously by the reader. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Tal Zussman <tz2294@columbia.edu> Reviewed-by: Jan Kara <jack@suse.cz>
Block device buffered reads and writes already pass through filemap_read() and iomap_file_buffered_write() respectively, both of which handle IOCB_DONTCACHE. Enable RWF_DONTCACHE for block device files by setting FOP_DONTCACHE in def_blk_fops. For CONFIG_BUFFER_HEAD=y, writeback goes through buffer_head's __bh_submit() which sets BIO_COMPLETE_IN_TASK on dropbehind folios. For CONFIG_BUFFER_HEAD=n, writeback goes through iomap which handles it via BIO_COMPLETE_IN_TASK on the ioend bio. This support is useful for databases that operate on raw block devices, among other userspace applications. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Tal Zussman <tz2294@columbia.edu> Reviewed-by: Jan Kara <jack@suse.cz>
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Pull request for series with
subject: block: enable RWF_DONTCACHE for block devices
version: 7
url: https://patchwork.kernel.org/project/linux-block/list/?series=1137152