linux-kernel - Re: [PATCH] target/user: Add daynmic growing data area featuresupport

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Date:   Mon, 27 Feb 2017 13:59:53 +0800
From:   Xiubo Li <lixiubo@...s.chinamobile.com>
To:     Andy Grover <agrover@...hat.com>, nab@...ux-iscsi.org,
        mchristi@...hat.com, shli@...nel.org
Cc:     hch@....de, sheng@...ker.org, namei.unix@...il.com,
        bart.vanassche@...disk.com, linux-scsi@...r.kernel.org,
        target-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jianfei Hu <hujianfei@...s.chinamobile.com>
Subject: Re: [PATCH] target/user: Add daynmic growing data area featuresupport

>>> Cool. This is a good approach for an initial patch but this raises
>>> concerns about efficiently managing kernel memory usage -- the data area
>>> grows but never shrinks, and total possible usage increases per
>>> backstore. (What if there are 1000?) Any ideas how we could also improve
>>> these aspects of the design? (Global TCMU data area usage limit?)
>> Two ways in my mind:
>>
>> The first:
>> How about by setting a threshold cmd(SHRINK cmd), something likes
>> the PAD cmd, to tell the userspace runner try to shrink the memories?
> Why should userspace need to know if the kernel is shrinking memory
> allocated to the data area? Userspace knows about memory described in
> iovecs for in-flight cmds, we shouldn't need its cooperation to free
> other allocated parts of the data area.
Yes.

> But, We likely don't want to release memory from the data area anyways
> while active, in any case. How about if we set a timer when active
> commands go to zero, and then reduce data area to some minimum if no new
> cmds come in before timer expires?

If I understand correctly: for example, we have 1G(as the minimum)
data area and all blocks have been allocated and mapped to runner's
vma, then we extern it to 1G + 256M as needed. When there have no
active cmds and after the timer expires, will it reduce the data area
back to 1G ? And then should it release the reduced 256M data area's
memories ?

If so, after kfree()ed the blocks' memories, it should also try to remove
all the ptes which are mapping this page(like using the try_to_umap()),
but something like try_to_umap() doesn't export for the modules.

Without ummaping the kfree()ed pages' ptes mentioned above, then
the reduced 256M vma space couldn't be reused again for the runner
process, because the runner has already do the mapping for the reduced
vma space to some old physical pages(won't trigger new page fault
again). Then there will be a hole, and the hole will be bigger and bigger.

Without ummaping the kfree()ed pages' ptes mentioned above, the
pages' reference count (page_ref_dec(), which _inc()ed in page fault)
couldn't be reduced back too.

>> When the runner get the SHRINK cmd, it will try to remmap uio0's ring
>> buffer(?). Then the kernel will get chance to shrink the memories....
>>
>> The second:
>> Try to extern the data area by using /dev/uio1, we could remmap the
>> uio1 device when need, so it will be easy to get a chance to shrink the
>> memories in uio1.
> Userspace should not need to remap the region in order for the kernel to
> free and unmap pages from the region.
>
> The only thing we need to watch out for is if blocks are referenced by
> in-flight cmds, we can't free them or userspace will segfault.
Yes, agree.

> So, if we
> wait until there are no in-flight cmds, then it follows that the kernel
> can free whatever it wants and userspace will not segfault.
So, the problem is how to ummap the kfree()ed pages' ptes.

BRs
Xiubo

> -- Andy
>