Re: [PATCH 1/2] Block layer filter - second version

[Matthew Wilcox]

On Wed, Oct 21, 2020 at 03:55:55PM +0300, Sergei Shtepa wrote:
> The 10/21/2020 14:44, Matthew Wilcox wrote:
> > I don't understand why O_DIRECT gets to bypass the block filter.  Nor do
> > I understand why anybody would place a block filter on the swap device.
> > But if somebody did place a filter on the swap device, why should swap
> > be able to bypass the filter?
> 
> Yes, intercepting the swap partition is absurd. But we can't guarantee
> that the filter won't intercept swap.
> 
> Swap operation is related to the memory allocation logic. If a swap on
> the block device are accessed during memory allocation from filter,
> a deadlock occurs. We can allow filters to occasionally shoot off their
> feet, especially under high load. But I think it's better not to do it.

We already have logic to prevent this in Linux.  Filters need to
call memalloc_noio_save() while they might cause swap to happen and
memalloc_noio_restore() once it's safe for them to cause swap again.

> "directly access" - it is not O_DIRECT. This means (I think) direct
> reading from the device file, like "dd if=/dev/sda1".
> As for intercepting direct reading, I don't know how to do the right thing.
> 
> The problem here is that in fs/block_dev.c in function __blkdev_direct_IO()
> uses the qc - value returned by the submit_bio() function.
> This value is used below when calling 
> blk_poll(bdev_get_queue(dev), qc, true).
> The filter cannot return a meaningful value of the blk_qc_t type when
> intercepting a request, because at that time it does not know which queue
> the request will fall into.
> 
> If function submit_bio() will always return BLK_QC_T_NONE - I think the
> algorithm of the __blk dev_direct_IO() will not work correctly.
> If we need to intercept direct access to a block device, we need to at
> least redo the __blkdev_direct_IO function, getting rid of blk_pool.
> I'm not sure it's necessary yet.

This isn't part of the block layer that I'm familiar with, so I can't
help solve this problem, but allowing O_DIRECT to bypass the block filter
is a hole that needs to be fixed before these patches can be considered.