Re: [PATCH] module: Replace kmap() with kmap_local_page()
From: Luis Chamberlain <mcgrof@kernel.org>
Date: 2022-07-18 22:19:59
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On Mon, Jul 18, 2022 at 02:26:45AM +0200, Fabio M. De Francesco wrote:
kmap() is being deprecated in favor of kmap_local_page(). Two main problems with kmap(): (1) It comes with an overhead as mapping space is restricted and protected by a global lock for synchronization and (2) it also requires global TLB invalidation when the kmap’s pool wraps and it might block when the mapping space is fully utilized until a slot becomes available.
Neat!
With kmap_local_page() the mappings are per thread, CPU local, can take page faults, and can be called from any context (including interrupts).
Yes but the mapping can only be accessed from within this thread and the thread is bound to the CPU while the mapping is active. So, what if we get a preemption during decompression here? What happens?
It is faster than kmap() in kernels with HIGHMEM enabled. Its use in module_gzip_decompress() and in module_xz_decompress() is safe and should be preferred. Therefore, replace kmap() with kmap_local_page().
While this churn is going on everywhere I was wondering why not go ahead and adopt kmap_local_folio() instead? Luis
quoted hunk ↗ jump to hunk
Tested on a QEMU/KVM x86_32 VM with 4GB RAM, booting kernels with HIGHMEM64GB enabled. Modules compressed with XZ or GZIP decompress properly. Suggested-by: Ira Weiny <ira.weiny@intel.com> Signed-off-by: Fabio M. De Francesco <redacted> --- kernel/module/decompress.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)diff --git a/kernel/module/decompress.c b/kernel/module/decompress.c index 2fc7081dd7c1..4d0bcb3d9e44 100644 --- a/kernel/module/decompress.c +++ b/kernel/module/decompress.c@@ -119,10 +119,10 @@ static ssize_t module_gzip_decompress(struct load_info *info, goto out_inflate_end; } - s.next_out = kmap(page); + s.next_out = kmap_local_page(page); s.avail_out = PAGE_SIZE; rc = zlib_inflate(&s, 0); - kunmap(page); + kunmap_local(s.next_out); new_size += PAGE_SIZE - s.avail_out; } while (rc == Z_OK);@@ -178,11 +178,11 @@ static ssize_t module_xz_decompress(struct load_info *info, goto out; } - xz_buf.out = kmap(page); + xz_buf.out = kmap_local_page(page); xz_buf.out_pos = 0; xz_buf.out_size = PAGE_SIZE; xz_ret = xz_dec_run(xz_dec, &xz_buf); - kunmap(page); + kunmap_local(xz_buf.out); new_size += xz_buf.out_pos; } while (xz_buf.out_pos == PAGE_SIZE && xz_ret == XZ_OK);-- 2.37.1