x86/mm/cpa: Prevent large page split when ftrace flips RW on kernel text
ftrace does not use text_poke() for enabling trace functionality. It uses its own mechanism and flips the whole kernel text to RW and back to RO. The CPA rework removed a loop based check of 4k pages which tried to preserve a large page by checking each 4k page whether the change would actually cover all pages in the large page. This resulted in endless loops for nothing as in testing it turned out that it actually never preserved anything. Of course testing missed to include ftrace, which is the one and only case which benefitted from the 4k loop. As a consequence enabling function tracing or ftrace based kprobes results in a full 4k split of the kernel text, which affects iTLB performance. The kernel RO protection is the only valid case where this can actually preserve large pages. All other static protections (RO data, data NX, PCI, BIOS) are truly static. So a conflict with those protections which results in a split should only ever happen when a change of memory next to a protected region is attempted. But these conflicts are rightfully splitting the large page to preserve the protected regions. In fact a change to the protected regions itself is a bug and is warned about. Add an exception for the static protection check for kernel text RO when the to be changed region spawns a full large page which allows to preserve the large mappings. This also prevents the syslog to be spammed about CPA violations when ftrace is used. The exception needs to be removed once ftrace switched over to text_poke() which avoids the whole issue. Fixes: 585948f4 ("x86/mm/cpa: Avoid the 4k pages check completely") Reported-by:Song Liu <songliubraving@fb.com> Signed-off-by:
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