On 3/1/22 00:11, David Hildenbrand wrote:
...
+EXPORT_SYMBOL(pin_user_page);
+
/*
* pin_user_pages_unlocked() is the FOLL_PIN variant of
* get_user_pages_unlocked(). Behavior is the same, except that this one sets
I assume that function will only get called on a page that has been
obtained by a previous pin_user_pages_fast(), correct?
Well, no. This is meant to be used in place of get_page(), for code that
knows that the pages will be released via unpin_user_page(). So there is
no special prerequisite there.
That might be problematic and possibly the wrong approach, depending on
*what* we're actually pinning and what we're intending to do with that.
My assumption would have been that this interface is to duplicate a pin
I see that I need to put more documentation here, so people don't have
to assume things... :)
on a page, which would be perfectly fine, because the page actually saw
a FOLL_PIN previously.
We're taking a pin on a page that we haven't obtained via FOLL_PIN if I
understand correctly. Which raises the questions, how do we end up with
the pages here, and what are we doing to do with them (use them like we
obtained them via FOLL_PIN?)?
If it's converting FOLL_GET -> FOLL_PIN manually, then we're bypassing
FOLL_PIN special handling in GUP code:
page = get_user_pages(FOLL_GET)
pin_user_page(page)
put_page(page)
No, that's not where this is going at all. The idea, which I now see
needs better documentation, is to handle file-backed pages. Only.
We're not converting from one type to another, nor are we doubling up.
We're just keeping the pin type consistent so that the vast block-
processing machinery can take pages in and handle them, then release
them at the end with bio_release_pages(), which will call
unpin_user_pages().
For anonymous pages, we'll bail out for example once we have
https://lkml.kernel.org/r/20220224122614.94921-14-david@xxxxxxxxxx
Because the conditions for pinned anonymous pages might no longer hold.
If we won't call pin_user_page() on anonymous pages, it would be fine.
We won't, and in fact, I should add WARN_ON_ONCE(PageAnon(page)) to
this function.
But then, I still wonder how we come up the "struct page" here.
From the file system. For example, the NFS-direct and fuse conversions
in the last patches show how that works.
Thanks for this feedback, this is very helpful.
thanks,
--
John Hubbard
NVIDIA
