Re: [PATCH v2] mm/filemap: Allow arch to request folio size for exec memory

On 15/02/2024 22:48, Andrew Morton wrote:

On Thu, 15 Feb 2024 15:40:59 +0000 Ryan Roberts [off-list ref] wrote:

quoted

Change the readahead config so that if it is being requested for an
executable mapping, do a synchronous read of an arch-specified size in a
naturally aligned manner.

Some nits:

Thanks for taking a look, Andrew!

quoted

--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h

@@ -1115,6 +1115,18 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
  */
 #define arch_wants_old_prefaulted_pte	cpu_has_hw_af

+/*
+ * Request exec memory is read into pagecache in at least 64K folios. The
+ * trade-off here is performance improvement due to storing translations more
+ * effciently in the iTLB vs the potential for read amplification due to reading

"efficiently"

ACK; will fix if there is a v3

quoted

+ * data from disk that won't be used. The latter is independent of base page
+ * size, so we set a page-size independent block size of 64K. This size can be
+ * contpte-mapped when 4K base pages are in use (16 pages into 1 iTLB entry),
+ * and HPA can coalesce it (4 pages into 1 TLB entry) when 16K base pages are in
+ * use.
+ */
+#define arch_wants_exec_folio_order() ilog2(SZ_64K >> PAGE_SHIFT)
+

To my eye, "arch_wants_foo" and "arch_want_foo" are booleans.  Either
this arch wants a particular treatment or it does not want it.

I suggest a better name would be "arch_exec_folio_order".

ACK; will fix if there is a v3

quoted

 static inline bool pud_sect_supported(void)
 {
 	return PAGE_SIZE == SZ_4K;

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index aab227e12493..6cdd145cbbb9 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -407,6 +407,18 @@ static inline bool arch_has_hw_pte_young(void)
 }
 #endif

+#ifndef arch_wants_exec_folio_order
+/*
+ * Returns preferred minimum folio order for executable file-backed memory. Must
+ * be in range [0, PMD_ORDER]. Negative value implies that the HW has no
+ * preference and mm will not special-case executable memory in the pagecache.
+ */

I think this comment contains material which would be useful above the
other arch_wants_exec_folio_order() implementation - the "must be in
range" part.  So I suggest all this material be incorporated into a
single comment which describes arch_wants_exec_folio_order().  Then
this comment can be removed entirely.  Assume the reader knows to go
seek the other definition for the commentary.

Hmm... The approach I've been taking for other arch-overridable helpers is to
put the API spec against the default implementation (i.e. here) then put
comments about the specific implementation against the override. If anything I
would prefer to formalize this comment into proper doc header comment and leave
it here (see for example set_ptes(), and in recent patches now in mm-unstable;
get_and_clear_full_ptes(), wrprotect_ptes(), etc).

I'll move all of this to the arm64 code if you really think that's the right
approach, but that's not my personal preference.

Thanks,
Ryan

quoted

+static inline int arch_wants_exec_folio_order(void)
+{
+	return -1;
+}
+#endif
+
 #ifndef arch_check_zapped_pte
 static inline void arch_check_zapped_pte(struct vm_area_struct *vma,
 					 pte_t pte)

diff --git a/mm/filemap.c b/mm/filemap.c
index 142864338ca4..7954274de11c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c

@@ -3118,6 +3118,25 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	}
 #endif

+	/*
+	 * Allow arch to request a preferred minimum folio order for executable
+	 * memory. This can often be beneficial to performance if (e.g.) arm64
+	 * can contpte-map the folio. Executable memory rarely benefits from
+	 * read-ahead anyway, due to its random access nature.

"readahead"

quoted

+	 */
+	if (vm_flags & VM_EXEC) {
+		int order = arch_wants_exec_folio_order();
+
+		if (order >= 0) {
+			fpin = maybe_unlock_mmap_for_io(vmf, fpin);
+			ra->size = 1UL << order;
+			ra->async_size = 0;
+			ractl._index &= ~((unsigned long)ra->size - 1);
+			page_cache_ra_order(&ractl, ra, order);
+			return fpin;
+		}
+	}
+
 	/* If we don't want any read-ahead, don't bother */
 	if (vm_flags & VM_RAND_READ)
 		return fpin;

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