On 30.09.20 16:39, James Bottomley wrote:

On Wed, 2020-09-30 at 13:27 +0300, Mike Rapoport wrote:

quoted

On Tue, Sep 29, 2020 at 05:15:52PM +0200, Peter Zijlstra wrote:

quoted

On Tue, Sep 29, 2020 at 05:58:13PM +0300, Mike Rapoport wrote:

quoted

On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:

quoted

It will drop them down to 4k pages. Given enough inodes, and
allocating only a single sekrit page per pmd, we'll shatter the
directmap into 4k.

Why? Secretmem allocates PMD-size page per inode and uses it as a
pool of 4K pages for that inode. This way it ensures that
__kernel_map_pages() is always called on PMD boundaries.

Oh, you unmap the 2m page upfront? I read it like you did the unmap
at the sekrit page alloc, not the pool alloc side of things.

Then yes, but then you're wasting gobs of memory. Basically you can
pin 2M per inode while only accounting a single page.

Right, quite like THP :)

I considered using a global pool of 2M pages for secretmem and
handing 4K pages to each inode from that global pool. But I've
decided to waste memory in favor of simplicity.

I can also add that the user space consumer of this we wrote does its
user pool allocation at a 2M granularity, so nothing is actually
wasted.

... for that specific user space consumer. (or am I missing something?)

-- 
Thanks,

David / dhildenb