linux-kernel - how to unmap pages in an anonymous mmap?

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Date:   Mon, 27 Feb 2017 11:32:48 -0800
From:   Andy Grover <agrover@...hat.com>
To:     Xiubo Li <lixiubo@...s.chinamobile.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>, linux-mm@...ck.org
Subject: how to unmap pages in an anonymous mmap?

On 02/26/2017 09:59 PM, Xiubo Li wrote:
>> 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.

Let's ask people who will know...

Hi linux-mm,

TCM-User (drivers/target/target_core_user.c) currently uses vmalloc()ed
memory to back a ring buffer that is mmaped by userspace.

We want to move to dynamically mapping pages into this region, and also
we'd like to unmap/free pages when idle. What's the right way to unmap?
I see unmap_mapping_range() but that mentions an underlying file, which
TCMU doesn't have. Or maybe zap_page_range()? But it's not exported.

Any advice?

Thanks in advance -- Regards -- Andy