Hi Balbir,
On Sat, Oct 27, 2018 at 09:21:02PM +1100, Balbir Singh wrote:
On Wed, Oct 24, 2018 at 07:13:50PM -0700, Joel Fernandes wrote:
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On Wed, Oct 24, 2018 at 10:57:33PM +1100, Balbir Singh wrote:
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+ pmd_t pmd;
+
+ new_ptl = pmd_lockptr(mm, new_pmd);
Looks like this is largely inspired by move_huge_pmd(), I guess a lot of
the code applies, why not just reuse as much as possible? The same comments
w.r.t mmap_sem helping protect against lock order issues applies as well.
I thought about this and when I looked into it, it seemed there are subtle
differences that make such sharing not worth it (or not possible).
Could you elaborate on them?
The move_huge_page function is defined only for CONFIG_TRANSPARENT_HUGEPAGE
so we cannot reuse it to begin with, since we have it disabled on our
systems. I am not sure if it is a good idea to split that out and refactor it
for reuse especially since our case is quite simple compared to huge pages.
There are also a couple of subtle differences between the move_normal_pmd and
the move_huge_pmd. Atleast 2 of them are:
1. We don't concern ourself with the PMD dirty bit, since the pages being
moved are normal pages and at the soft-dirty bit accounting is at the PTE
level, since we are not moving PTEs, we don't need to do that.
2. The locking is simpler as Kirill pointed, pmd_lock cannot fail however
__pmd_trans_huge_lock can.
I feel it is not super useful to refactor move_huge_pmd to support our case
especially since move_normal_pmd is quite small, so IMHO the benefit of code
reuse isn't there very much.
Do let me know your thoughts and thanks for your interest in this.
thanks,
- Joel