Le 17/09/2024 à 09:31, Anshuman Khandual a écrit :
This series converts all generic page table entries direct derefences via
pxdp_get() based helpers extending the changes brought in via the commit
c33c794828f2 ("mm: ptep_get() conversion"). First it does some platform
specific changes for m68k and x86 architecture.
This series has been build tested on multiple architecture such as x86,
arm64, powerpc, powerpc64le, riscv, and m68k etc.
Seems like this series imply sub-optimal code with unnecessary reads.
Lets take a simple exemple : function mm_find_pmd() in mm/rmap.c
On a PPC32 platform (2 level pagetables):
Before the patch:
00001b54 <mm_find_pmd>:
1b54: 80 63 00 18 lwz r3,24(r3)
1b58: 54 84 65 3a rlwinm r4,r4,12,20,29
1b5c: 7c 63 22 14 add r3,r3,r4
1b60: 4e 80 00 20 blr
Here, the function reads mm->pgd, then calculates and returns a pointer
to the PMD entry corresponding to the address.
After the patch:
00001b54 <mm_find_pmd>:
1b54: 81 23 00 18 lwz r9,24(r3)
1b58: 54 84 65 3a rlwinm r4,r4,12,20,29
1b5c: 7d 49 20 2e lwzx r10,r9,r4 <= useless read
1b60: 7c 69 22 14 add r3,r9,r4
1b64: 7d 49 20 2e lwzx r10,r9,r4 <= useless read
1b68: 7d 29 20 2e lwzx r9,r9,r4 <= useless read
1b6c: 4e 80 00 20 blr
Here, the function also reads mm->pgd and still calculates and returns a
pointer to the PMD entry corresponding to the address. But in addition
to that it reads three times that entry while doing nothing at all with
the value read.
On PPC32, PMD/PUD/P4D are single entry tables folded into the
corresponding PGD entry, it is therefore pointless to read the
intermediate entries.
Christophe
