Re: [PATCH v3 1/2] mm: Abstract THP allocation
From: Dev Jain
Date: Wed Sep 11 2024 - 08:03:14 EST
On 9/11/24 14:57, David Hildenbrand wrote:
On 11.09.24 08:55, Dev Jain wrote:
In preparation for the second patch, abstract away the THP allocation
logic present in the create_huge_pmd() path, which corresponds to the
faulting case when no page is present.
There should be no functional change as a result of applying
this patch.
Hi,
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
mm/huge_memory.c | 110 +++++++++++++++++++++++++++++------------------
1 file changed, 67 insertions(+), 43 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 67c86a5d64a6..b96a1ff2bf40 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -943,47 +943,88 @@ unsigned long thp_get_unmapped_area(struct file
*filp, unsigned long addr,
}
EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
-static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
- struct page *page, gfp_t gfp)
+static struct folio *pmd_thp_fault_alloc(gfp_t gfp, struct
vm_area_struct *vma,
+ unsigned long haddr, unsigned long addr)
I suggest calling this something like "vma_alloc_anon_folio_pmd()"?
Then it's more consistent with vma_alloc_folio().
Also, likely we should just only pass in "addr" and calculate "haddr"
ourselves, it's cheap and reduces the number of function parameters.
Makes sense, thanks.
{
- struct vm_area_struct *vma = vmf->vma;
- struct folio *folio = page_folio(page);
- pgtable_t pgtable;
- unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
- vm_fault_t ret = 0;
+ const int order = HPAGE_PMD_ORDER;
+ struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr,
true);
- VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+ if (unlikely(!folio)) {
+ count_vm_event(THP_FAULT_FALLBACK);
+ count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+ goto out;
+ }
+ VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
folio_put(folio);
count_vm_event(THP_FAULT_FALLBACK);
count_vm_event(THP_FAULT_FALLBACK_CHARGE);
- count_mthp_stat(HPAGE_PMD_ORDER,
MTHP_STAT_ANON_FAULT_FALLBACK);
- count_mthp_stat(HPAGE_PMD_ORDER,
MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
- return VM_FAULT_FALLBACK;
+ count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK);
+ count_mthp_stat(order, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
+ goto out;
}
folio_throttle_swaprate(folio, gfp);
- pgtable = pte_alloc_one(vma->vm_mm);
- if (unlikely(!pgtable)) {
- ret = VM_FAULT_OOM;
- goto release;
- }
-
- folio_zero_user(folio, vmf->address);
+ folio_zero_user(folio, addr);
/*
* The memory barrier inside __folio_mark_uptodate makes sure that
* folio_zero_user writes become visible before the set_pmd_at()
* write.
*/
__folio_mark_uptodate(folio);
+out:
+ return folio;
+}
+
+static void __pmd_thp_fault_success_stats(struct vm_area_struct *vma)
+{
+ count_vm_event(THP_FAULT_ALLOC);
+ count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_ALLOC);
+ count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
+}
Why isn't that moved into map_pmd_thp()
Note that in this patch you do:
map_pmd_thp(folio, vmf, vma, haddr);
spin_unlock(vmf->ptl);
__pmd_thp_fault_success_stats(vma);
But in patch #2
map_pmd_thp(folio, vmf, vma, haddr);
__pmd_thp_fault_success_stats(vma);
goto unlock;
release:
folio_put(folio);
unlock:
spin_unlock(vmf->ptl);
Yes, while writing it I knew about this inconsistency, but I wanted
to reduce latency by dropping the lock before. But in do_huge_zero_wp_pmd(),
I couldn't figure out a way to call the stat function after dropping the
lock,
without I guess, introducing too many labels and goto jumps and the
like. In the
current code, the lock gets dropped first.
Please make that consistent, meaning:
1) Inline __pmd_thp_fault_success_stats() into map_pmd_thp(). No need
to have the separated out.
2) Either do the PTL unlocking in __pmd_thp_fault_success_stats() or in
the caller. In the caller is likely easiest. Adjusting the counters
should be cheap, if not we could revisit this later with real data.
I will then call it in map_pmd_thp(), that is cleaner...I did find
occurrences
of these stat computations after taking the lock, for example, in
do_swap_page():
count_vm_event(PGMAJFAULT)
so I guess it should be alright.
+
+static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
+ struct vm_area_struct *vma, unsigned long haddr)
+{
+ pmd_t entry;
+
+ entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
+ entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+ folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
+ folio_add_lru_vma(folio, vma);
+ set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
+ update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
It's quite weird to see a mixture of haddr and vmf->address, and
likely this mixture is wrong or not not required.
Looking at arc's update_mmu_cache_pmd() implementation, I cannot see
how passing in the unaligned address would do the right thing. But
maybe arc also doesn't trigger that code path ... who knows :)
Staring at some other update_mmu_cache_pmd() users, it's quite
inconsistent. Primarily only do_huge_pmd_numa_page() and
__do_huge_pmd_anonymous_page() use the unaligned address. The others
seem to use the aligned address ... as one would expect when modifying
a PMD.
I suggest to change this function to *not* pass in the vmf, and rename
it to something like:
static void folio_map_anon_pmd(struct folio *folio, struct
vm_area_struct *vma, pmd_t *pmd, unsigned long haddr)
Then use haddr also to do the update_mmu_cache_pmd().
The code I changed, already was passing vmf->address to
update_mmu_cache_pmd().
I did not change any of the haddr and vmf->address semantics, so really
can't comment
on this.
I agree with the name change; vmf will be required for
set_pmd_at(vmf->pmd), so I should
just pass pmd?
+ add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
+ mm_inc_nr_ptes(vma->vm_mm);
+}
+
+static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ struct folio *folio;
+ pgtable_t pgtable;
+ unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+ vm_fault_t ret = 0;
+ gfp_t gfp = vma_thp_gfp_mask(vma);
Nit: While at it, try to use reverse christmas-tree where possible,
makes things more reasible. You could make haddr const.
struct vm_area_struct *vma = vmf->vma;
unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
gfp_t gfp = vma_thp_gfp_mask(vma);
struct folio *folio;
vm_fault_t ret = 0;
Okay.
...
+
+ folio = pmd_thp_fault_alloc(gfp, vma, haddr, vmf->address);
+ if (unlikely(!folio)) {
+ ret = VM_FAULT_FALLBACK;
+ goto release;
+ }
+
+ pgtable = pte_alloc_one(vma->vm_mm);
+ if (unlikely(!pgtable)) {
+ ret = VM_FAULT_OOM;
+ goto release;
+ }
vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+
Nit Unrelated change.
Are you asking me to align this line with the below line?
if (unlikely(!pmd_none(*vmf->pmd))) {
goto unlock_release;