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.
"
[1]
https://lore.kernel.org/all/f1783ff0-65bd-4b2b-8952-52b6822a0835@xxxxxxxxxx/T/#u
