I've wondered about this for a while. In ib_umem_get, there is a
call to get_user_pages that does the work of virtual to physical
translation and increasing the ref counts. It is always invoked
with write == 1, even if cmd.access_flags == 0 (read only
registration).
This is fine for anonymous private memory, or writeable shared
memory. But consider pinning a read-only section of memory, such as
shared read-only data or text segment, or a file mapping of a file
that was opened O_RDONLY. Having write == 1 there forces a full
copy of all these pages.
The force argument is explicitly set to 1 only when access_flags
does not specify write access, giving gup permission to do the
copy-on-write, essentially. That seems correct, but always setting
write to 1 has me confused.
Is there some IB semantic reason for forcing the registered pages to
be writable?
-- Pete
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