On Fri, Aug 11, 2023 at 08:30:16PM +0500, Muhammad Usama Anjum wrote:
On 8/11/23 6:32 PM, Michał Mirosław wrote:
On Fri, Aug 11, 2023 at 05:02:44PM +0500, Muhammad Usama Anjum wrote:
Now we are walking the entire range walk_page_range(). We don't break loop when we get -ENOSPC as this error may only mean that the temporary buffer is full. So we need check if max pages have been found or output buffer is full or ret is 0 or any other error. When p.arg.vec_len = 1 is end condition as the last entry is in cur. As we have walked over the entire range, cur must be full after which the walk returned.
So current condition is necessary. I've double checked it. I'll change it to `p.arg.vec_len == 1`.
If we have walked the whole range, then the loop will end anyway due to `walk_start < walk_end` not held in the `for()`'s condition.
Sorry, for not explaining to-the-point. Why would we walk the entire range when we should recognize that the output buffer is full and break the loop?
I've test cases written for this case. If I remove `p.arg.vec_len == 1` check, there is infinite loop for walking. So we are doing correct thing here.
It seems there is a bug somewhere then. I'll take a look at v29.
[...]
+/*
- struct pm_scan_arg - Pagemap ioctl argument
- @size: Size of the structure
- @flags: Flags for the IOCTL
- @start: Starting address of the region
- @end: Ending address of the region
- @walk_end Address where the scan stopped (written by kernel).
walk_end == end informs that the scan completed on entire range.Can we ensure this holds also for the tagged pointers?
No, we cannot.
So this need explanation in the comment here. (Though I'd still like to know how the address tags are supposed to be used from someone that knows them.)
I've looked at some documentations (presentations/talks) about tags. Tags is more like userspace feature. Kernel should just ignore them for our use case. I'll add comment.
Kernel does ignore them when reading, but what about returning a tagged pointer? How that should work? In case of `walk_end` we can safely copy the tag from `end` or `start` when we return exactly on of those. But what about other addresses? When fed back as `start` any tag will work, so the question is only what to do with pointers in the middle? We can clear those of course - this should be mentioned in the doc - so userspace always gets a predictable value (note: 'predictable' does not require treating `start` and `end` the same way as addresses between them, just that what happens is well defined). (I think making `walk_end` == `end` work regardless of pointer tagging will make userspace happier, but I guess doc will also make it workable. And I'm repeating myself. ;-)
Best Regards Michał Mirosław