[PATCH 1/2] iohelper: skip lseek() and ftruncate() on block devices

[Simon Rowe]

Missed to answer part of your question. I'm using an NBD device backed by a userspace process using nbdkit,

Simon
________________________________
From: Simon Rowe <simon.rowe at nutanix.com>
Sent: 23 August 2021 16:15
To: Michal Prívozník <mprivozn at redhat.com>; libvir-list at redhat.com <libvir-list at redhat.com>
Subject: Re: [PATCH 1/2] iohelper: skip lseek() and ftruncate() on block devices

I'm using 5.4.109. I'll re-submit with changes to the second patch shortly,

thanks
Simon
________________________________
From: Michal Prívozník <mprivozn at redhat.com>
Sent: 23 August 2021 16:04
To: Simon Rowe <simon.rowe at nutanix.com>; libvir-list at redhat.com <libvir-list at redhat.com>
Subject: Re: [PATCH 1/2] iohelper: skip lseek() and ftruncate() on block devices

On 8/23/21 4:38 PM, Simon Rowe wrote:
> O_DIRECT is under control of the caller (though the --bypass-cache option). I am running in a constrained memory environment and when I don't use O_DIRECT the dump stalls at 5% load av hits 41 and the system becomes unresponsive.

Fair enough. I thought that block devices are always opened without
poisoning host cache. If they aren't then you patch makes sense.
But, what is your kernel version? Hopefully it's not too old. Is it a
special block device or just some regular disk?

Looking into the kernel code, it does implement O_DIRECT for block
devices:

kernel.git $ git grep -npC10 "\.direct_IO" -- fs/block_dev.c
fs/block_dev.c-1680-}
fs/block_dev.c-1681-
fs/block_dev.c=1682=static const struct address_space_operations def_blk_aops = {
fs/block_dev.c-1683-    .set_page_dirty = __set_page_dirty_buffers,
fs/block_dev.c-1684-    .readpage       = blkdev_readpage,
fs/block_dev.c-1685-    .readahead      = blkdev_readahead,
fs/block_dev.c-1686-    .writepage      = blkdev_writepage,
fs/block_dev.c-1687-    .write_begin    = blkdev_write_begin,
fs/block_dev.c-1688-    .write_end      = blkdev_write_end,
fs/block_dev.c-1689-    .writepages     = blkdev_writepages,
fs/block_dev.c:1690:    .direct_IO      = blkdev_direct_IO,
fs/block_dev.c-1691-    .migratepage    = buffer_migrate_page_norefs,
fs/block_dev.c-1692-    .is_dirty_writeback = buffer_check_dirty_writeback,
fs/block_dev.c-1693-};
fs/block_dev.c-1694-
fs/block_dev.c-1695-#define     BLKDEV_FALLOC_FL_SUPPORTED                                      \
fs/block_dev.c-1696-            (FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |           \
fs/block_dev.c-1697-             FALLOC_FL_ZERO_RANGE | FALLOC_FL_NO_HIDE_STALE)
fs/block_dev.c-1698-
fs/block_dev.c-1699-static long blkdev_fallocate(struct file *file, int mode, loff_t start,
fs/block_dev.c-1700-                         loff_t len)

So it seems like your patch was correct after all. Would you like to
resend this in v2 (I understand that patch 2/2 needs a rework) or do you
want me to push this one right away?

Michal

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