On 10/23/19 4:55 AM, Mike Kravetz wrote:
On 10/22/19 12:09 AM, Piotr Sarna wrote:
On 10/21/19 7:17 PM, Mike Kravetz wrote:
On 10/15/19 4:37 PM, Mike Kravetz wrote:
On 10/15/19 3:50 AM, Michal Hocko wrote:
On Tue 15-10-19 11:01:12, Piotr Sarna wrote:
With hugetlbfs, a common pattern for mapping anonymous huge pages
is to create a temporary file first.
Really? I though that this is normally done by shmget(SHM_HUGETLB) or
mmap(MAP_HUGETLB). Or maybe I misunderstood your definition on anonymous
huge pages.
Currently libraries like
libhugetlbfs and seastar create these with a standard mkstemp+unlink
trick,
I would guess that much of libhugetlbfs was writen before MAP_HUGETLB
was implemented. So, that is why it does not make (more) use of that
option.
The implementation looks to be straight forward. However, I really do
not want to add more functionality to hugetlbfs unless there is specific
use case that needs it.
It was not my intention to shut down discussion on this patch. I was just
asking if there was a (new) use case for such a change. I am checking with
our DB team as I seem to remember them using the create/unlink approach for
hugetlbfs in one of their upcoming models.
Is there a new use case you were thinking about?
Oh, I indeed thought it was a shutdown. The use case I was thinking about was in Seastar, where the create+unlink trick is used for creating temporary files (in a generic way, not only for hugetlbfs). I simply intended to migrate it to a newer approach - O_TMPFILE. However,
for the specific case of hugetlbfs it indeed makes more sense to skip it and use mmap's MAP_HUGETLB, so perhaps it's not worth it to patch a perfectly good and stable file system just to provide a semi-useful flag support. My implementation of tmpfile for hugetlbfs is straightforward indeed, but the MAP_HUGETLB argument made me realize that it may not be worth the trouble - especially that MAP_HUGETLB is here since 2.6 and O_TMPFILE was introduced around v3.11, so the mmap way looks more portable.
tldr: I'd be very happy to get my patch accepted, but the use case I had in mind can be easily solved with MAP_HUGETLB, so I don't insist.
If you really are after something like 'anonymous memory' for Seastar,
then MAP_HUGETLB would be the better approach.
Just to clarify - my original goal was to migrate Seastar's temporary
file implementation (which is fs-agnostic, based on descriptors) from
the current create+unlink to O_TMPFILE, for robustness. One of the
internal usages of this generic mechanism was to create a tmpfile on
hugetlbfs and that's why I sent this patch. However, this particular
internal usage can be easily switched to more portable MAP_HUGETLB,
which will also mean that the generic tmpfile implementation will not be
used internally for hugetlbfs anymore.
There *may* still be value in being able to support hugetlbfs once
Seastar's tmpfile implementation migrates to O_TMPFILE, since the
library offers creating temporary files in its public API, but there's
no immediate use case I can apply it to.
I'm still checking with Oracle DB team as they may have a use for O_TMPFILE
in an upcoming release. In their use case, they want an open fd to work with.
If it looks like they will proceed in this direction, we can work to get
your patch moved forward.
Thanks,
Great, if it turns out that my patch helps anyone with their O_TMPFILE
usage, I'd be very glad to see it merged.
