Kewl :)
Let me know if you have more questions.
On Wed, Jun 13, 2012 at 3:20 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
> On 2012-06-13 15:03:42, Thieu Le wrote:
>> On Wed, Jun 13, 2012 at 2:17 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> wrote:
>> > On Wed, Jun 13, 2012 at 11:53 AM, Thieu Le <thieule@xxxxxxxxxx> wrote:
>> >>
>> >> Hi Tyler, I believe the performance improvement from the async
>> >> interface comes from the ability to fully utilize the crypto
>> >> hardware.
>> >>
>> >> Firstly, being able to submit multiple outstanding requests fills
>> >> the crypto engine pipeline which allows it to run more efficiently
>> >> (ie. minimal cycles are wasted waiting for the next crypto request).
>> >> This perf improvement is similar to network transfer efficiency.
>> >> Sending a 1GB file via 4K packets synchronously is not going to
>> >> fully saturate a gigabit link but queuing a bunch of 4K packets to
>> >> send will.
>> >
>> > Ok, it is clicking for me now. Additionally, I imagine that the async
>> > interface helps in the multicore/multiprocessor case.
>> >
>> >> Secondly, if you have crypto hardware that has multiple crypto
>> >> engines, then the multiple outstanding requests allow the crypto
>> >> hardware to put all of those engines to work.
>> >>
>> >> To answer your question about page_crypt_req, it is used to track
>> >> all of the extent_crypt_reqs for a particular page. When we write a
>> >> page, we break the page up into extents and encrypt each extent.
>> >> For each extent, we submit the encrypt request using
>> >> extent_crypt_req. To determine when the entire page has been
>> >> encrypted, we create one page_crypt_req and associates the
>> >> extent_crypt_req to the page by incrementing
>> >> page_crypt_req::num_refs. As the extent encrypt request completes,
>> >> we decrement num_refs. The entire page is encrypted when num_refs
>> >> goes to zero, at which point, we end the page writeback.
>> >
>> > Alright, that is what I had understood from reviewing the code. No
>> > surprises there.
>> >
>> > What I'm suggesting is to do away with the page_crypt_req and simply have
>> > ecryptfs_encrypt_page_async() keep track of the extent_crypt_reqs for
>> > the page it is encrypting. Its prototype would look like this:
>> >
>> > int ecryptfs_encrypt_page_async(struct page *page);
>> >
>> > An example of how it would be called would be something like this:
>> >
>> > static int ecryptfs_writepage(struct page *page, struct writeback_control *wbc)
>> > {
>> > int rc = 0;
>> >
>> > /*
>> > * Refuse to write the page out if we are called from reclaim context
>> > * since our writepage() path may potentially allocate memory when
>> > * calling into the lower fs vfs_write() which may in turn invoke
>> > * us again.
>> > */
>> > if (current->flags & PF_MEMALLOC) {
>> > redirty_page_for_writepage(wbc, page);
>> > goto out;
>> > }
>> >
>> > set_page_writeback(page);
>> > rc = ecryptfs_encrypt_page_async(page);
>> > if (unlikely(rc)) {
>> > ecryptfs_printk(KERN_WARNING, "Error encrypting "
>> > "page (upper index [0x%.16lx])\n", page->index);
>> > ClearPageUptodate(page);
>> > SetPageError(page);
>> > } else {
>> > SetPageUptodate(page);
>> > }
>> > end_page_writeback(page);
>> > out:
>> > unlock_page(page);
>> > return rc;
>> > }
>>
>> Will this make ecryptfs_encrypt_page_async() block until all of the
>> extents are encrypted and written to the lower file before returning?
>>
>> In the current patch, ecryptfs_encrypt_page_async() returns
>> immediately after the extents are submitted to the crypto layer.
>> ecryptfs_writepage() will also return before the encryption and write
>> to the lower file completes. This allows the OS to start writing
>> other pending pages without being blocked.
>
> Ok, now I see the source of my confusion. The wait_for_completion()
> added in ecryptfs_encrypt_page() was throwing me off. I initially
> noticed that and didn't realize that wait_for_completion() was *not*
> being called in ecryptfs_writepage().
>
> I hope to give the rest of the patch a thorough review by the end of the
> week. Thanks for your help!
>
> Tyler
>
>>
>>
>> >
>> >
>> >> We can get rid of page_crypt_req if we can guarantee that the extent
>> >> size and page size are the same.
>> >
>> > We can't guarantee that but that doesn't matter because
>> > ecryptfs_encrypt_page_async() already handles that problem. Its caller doesn't
>> > care if the extent size is less than the page size.
>> >
>> > Tyler
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