On 9/22/2022 1:58 PM, Alexander Duyck wrote:
On Thu, Sep 22, 2022 at 1:07 PM Anirudh Venkataramanan
<anirudh.venkataramanan@xxxxxxxxx> wrote:
Following Fabio's patches, I made similar changes for e1000/e1000e and
submitted them to IWL [1].
Yesterday, Ira Weiny pointed me to some feedback from Dave Hansen on the
use of page_address() [2]. My understanding of this feedback is that
it's safer to use kmap_local_page() instead of page_address(), because
you don't always know how the underlying page was allocated.
This approach (of using kmap_local_page() instead of page_address())
makes sense to me. Any reason not to go this way?
[1]
https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-1-anirudh.venkataramanan@xxxxxxxxx/
https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20220919180949.388785-2-anirudh.venkataramanan@xxxxxxxxx/
[2]
https://lore.kernel.org/lkml/5d667258-b58b-3d28-3609-e7914c99b31b@xxxxxxxxx/
Ani
For the two patches you referenced the driver is the one allocating
the pages. So in such a case the page_address should be acceptable.
Specifically we are falling into alloc_page(GFP_ATOMIC) which should
fall into the first case that Dave Hansen called out.
Right. However, I did run into a case in the chelsio inline crypto
driver where it seems like the pages are allocated outside the driver.
In such cases, kmap_local_page() would be the right approach, as the
driver can't make assumptions on how the page was allocated.
... and this makes me wonder why not just use kmap_local_page() even in
cases where the page allocation was done in the driver. IMO, this is
simpler because
a) you don't have to care how a page was allocated. kmap_local_page()
will create a temporary mapping if required, if not it just becomes a
wrapper to page_address().
b) should a future patch change the allocation to be from highmem, you
don't have to change a bunch of page_address() calls to be
kmap_local_page().
Is using page_address() directly beneficial in some way?
Ani
