On 2024/7/5 16:42, Ryan Roberts wrote:
On 05/07/2024 04:01, Baolin Wang wrote:
On 2024/7/4 22:46, Bang Li wrote:
Hi Bao lin,
On 2024/7/4 19:15, Baolin Wang wrote:
+
+ /*
+ * Only allow inherit orders if the top-level value is 'force', which
+ * means non-PMD sized THP can not override 'huge' mount option now.
+ */
+ if (shmem_huge == SHMEM_HUGE_FORCE)
+ return READ_ONCE(huge_shmem_orders_inherit);
I vaguely recall that we originally discussed that trying to set 'force' on the
top level control while any per-size controls were set to 'inherit' would be an
error, and trying to set 'force' on any per-size control except the PMD-size
would be an error?
Right.
I don't really understand this logic. Shouldn't we just be looking at the
per-size control settings (or the top-level control as a proxy for every
per-size control that has 'inherit' set)?
‘force’ will apply the huge orders for anon shmem and tmpfs, so now we only
allow pmd-mapped THP to be forced. We should not look at per-size control
settings for tmpfs now (mTHP for tmpfs will be discussed in future).
Then for tmpfs, which doesn't support non-PMD-sizes yet, we just always use the
PMD-size control for decisions.
I'm also really struggling with the concept of shmem_is_huge() existing along
side shmem_allowable_huge_orders(). Surely this needs to all be refactored into
shmem_allowable_huge_orders()?
I understood. But now they serve different purposes: shmem_is_huge() will be
used to check the huge orders for the top level, for *tmpfs* and anon shmem;
whereas shmem_allowable_huge_orders() will only be used to check the per-size
huge orders for anon shmem (excluding tmpfs now). However, as I plan to add
mTHP support for tmpfs, I think we can perform some cleanups.
Please count me in, I'd be happy to contribute to the cleanup and enhancement
process if I can.
Good. If you have time, I think you can look at the shmem khugepaged issue from
the previous discussion [1], which I don't have time to look at now.
"
(3) khugepaged
khugepaged needs to handle larger folios properly as well. Until fixed,
using smaller THP sizes as fallback might prohibit collapsing a
PMD-sized THP later. But really, khugepaged needs to be fixed to handle
that.
"
khugepaged can already collapse "folios of any order less then PMD-size" to
PMD-size, for anon memory. Infact I modified the selftest to be able to test
that in commit 9f0704eae8a4 ("selftests/mm/khugepaged: enlighten for multi-size
THP"). I'd be surprised if khugepaged can't alreay handle the same for shmem?
I did not test this, but from the comment in hpage_collapse_scan_file(),
seems that compacting small mTHP into a single PMD-mapped THP is not
supported yet.
/*
* TODO: khugepaged should compact smaller compound pages
* into a PMD sized page
*/
if (folio_test_large(folio)) {
result = folio_order(folio) == HPAGE_PMD_ORDER &&
folio->index == start
/* Maybe PMD-mapped */
? SCAN_PTE_MAPPED_HUGEPAGE
: SCAN_PAGE_COMPOUND;
/*
* For SCAN_PTE_MAPPED_HUGEPAGE, further processing
* by the caller won't touch the page cache, and so
* it's safe to skip LRU and refcount checks before
* returning.
*/
break;
}
Although the test will definitely want to be extended to test it.
Right.
