On 4/11/23 2:29 PM, Michał Mirosław wrote:
On Fri, 7 Apr 2023 at 13:11, Muhammad Usama Anjum usama.anjum@collabora.com wrote:
On 4/7/23 3:14 PM, Michał Mirosław wrote:
On Fri, 7 Apr 2023 at 12:04, Muhammad Usama Anjum usama.anjum@collabora.com wrote:
On 4/7/23 12:34 PM, Michał Mirosław wrote:
On Thu, 6 Apr 2023 at 23:04, Muhammad Usama Anjum usama.anjum@collabora.com wrote:
On 4/7/23 1:00 AM, Michał Mirosław wrote: > On Thu, 6 Apr 2023 at 19:58, Muhammad Usama Anjum > usama.anjum@collabora.com wrote:
[...]
>>>> + /* >>>> + * Allocate smaller buffer to get output from inside the page walk >>>> + * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As >>>> + * we want to return output to user in compact form where no two >>>> + * consecutive regions should be continuous and have the same flags. >>>> + * So store the latest element in p.cur between different walks and >>>> + * store the p.cur at the end of the walk to the user buffer. >>>> + */ >>>> + p.vec = kmalloc_array(p.vec_len, sizeof(struct page_region), >>>> + GFP_KERNEL); >>>> + if (!p.vec) >>>> + return -ENOMEM; >>>> + >>>> + walk_start = walk_end = start; >>>> + while (walk_end < end && !ret) { >>> >>> The loop will stop if a previous iteration returned ENOSPC (and the >>> error will be lost) - is it intended? >> It is intentional. -ENOSPC means that the user buffer is full even though >> there was more memory to walk over. We don't treat this error. So when >> buffer gets full, we stop walking over further as user buffer has gotten >> full and return as success. > > Thanks. What's the difference between -ENOSPC and > PM_SCAN_FOUND_MAX_PAGES? They seem to result in the same effect (code > flow). -ENOSPC --> user buffer has been filled completely PM_SCAN_FOUND_MAX_PAGES --> max_pages have been found, user buffer may still have more space
What is the difference in code behaviour when those two cases are compared? (I'd expect none.)
There is difference: We add data to user buffer. If it succeeds with return code 0, we engage the WP. If it succeeds with PM_SCAN_FOUND_MAX_PAGES, we still engage the WP. But if we get -ENOSPC, we don't perform engage as the data wasn't added to the user buffer.
Thanks! I see it now. I see a few more corner cases here:
- If we did engage WP but fail to copy the vector we return -EFAULT
but the WP is already engaged. I'm not sure this is something worth guarding against, but documenting that would be helpful I think.
Sure.
- If uffd_wp_range() fails, but we have already processed pages
earlier, we should treat the error like ENOSPC and back out the failed range (the earier changes would be lost otherwise).
Backing out is easier to do for hugepages. But for normal pages, we'll have to write some code to find where the current data was added (in cur or in vec) and back out from that. I'll have to write some more code to avoid the side-effects as well.
If I read the code correctly, the last page should always be in `cur` and on failure only a single page is needed to be backed out. Did I miss something?
I'm leaving using uffd_wp_range() in next revision as it is costing performance. This will not be needed in next revision.
But aren't we going over-engineering here? Error occurred and we are trying to keep the previously generated correct data and returning successfully still to the user? I don't think we should do this. An error is error. We should return the error simply even if the memory flags would get lost. We don't know what caused the error in uffd_wp_range(). Under normal situation, we there shouldn't have had error.
In this case it means that on (intermittent) allocation error we get inconsistent or non-deterministic results. I wouldn't want to be the one debugging this later - I'd prefer either the syscall be "exception-safe" (give consistent and predictable output) or kill the process.
Best Regards Michał Mirosław