On 8/22/20 11:57 PM, John Hubbard wrote:
On 8/22/20 11:25 PM, Christoph Hellwig wrote:
On Fri, Aug 21, 2020 at 09:20:58PM -0700, John Hubbard wrote:
Add a new BIO_FOLL_PIN flag to struct bio, whose "short int" flags field
was full, thuse triggering an expansion of the field from 16, to 32
bits. This allows for a nice assertion in bio_release_pages(), that the
bio page release mechanism matches the page acquisition mechanism.
Set BIO_FOLL_PIN whenever pin_user_pages_fast() is used, and check for
BIO_FOLL_PIN before using unpin_user_page().
When would the flag not be set when BIO_NO_PAGE_REF is not set?
Well, I don't *think* you can get there. However, I've only been studying
bio/block for a fairly short time, and the scattering of get_page() and
put_page() calls in some of the paths made me wonder if, for example,
someone was using get_page() to acquire ITER_BVEC or ITER_KVEC via
get_page(), and release them via bio_release_pages(). It's hard to tell.
It seems like that shouldn't be part of the design. I'm asserting that
it isn't, with this new flag. But if you're sure that this assertion is
unnecessary, then let's just drop this patch, of course.
Also, I should have done a few more subsystem conversions, before
concluding that BIO_FOLL_PIN was a good idea. Now, as I'm working through mopping
up those other subsystems, I see that nfs/direct.c for example does not have access
to a bio instance, and so the whole thing is not really a great move, at least not
for adding to the iov_iter_pin_user_pages*() APIs.
Let's just drop this patch, after all.
thanks,
--
John Hubbard
NVIDIA
