On Fri, Oct 11, 2024 at 11:22:37PM +0000, Ackerley Tng wrote:
> With the addition of the chg parameter, vma_has_reserves() no longer
> just determines whether the vma has reserves.
>
> The comment in the vma->vm_flags & VM_NORESERVE block indicates that
> this function actually computes whether or not the reserved count
> should be decremented.
>
> This refactoring also takes into account the allocation's request
> parameter avoid_reserve, which helps to further simplify the calling
> function alloc_hugetlb_folio().
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
I wonder if this patch could be merged with the 3rd, IIUC this can
fundamentally be seen as a movement of what patch 3 was removed.
Furthermore, I do feel like should_use_hstate_resv() could be redundant on
its own on many things.
Let me try to justify. Firstly, after 3 patches applied, now it looks like
this (I removed all comments to make things shorter..):
static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg,
bool avoid_reserve)
{
if (avoid_reserve)
return false;
if (vma->vm_flags & VM_NORESERVE) {
if (vma->vm_flags & VM_MAYSHARE && chg == 0)
return true;
else
return false;
}
if (vma->vm_flags & VM_MAYSHARE) {
if (chg)
return false;
else
return true;
}
if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
if (chg)
return false;
else
return true;
}
return false;
}
Then let's look at chg==0 processing all above: what does it say? I
suppose it means "we don't need another global reservation". It means if
chg==0 we always will use an existing reservation. From math POV it also
is true, so it can already be moved out ahead, IIUC, like this:
static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg,
bool avoid_reserve)
{
if (avoid_reserve)
return false;
if (chg == 0)
return true;
if (vma->vm_flags & VM_NORESERVE)
return false;
if (vma->vm_flags & VM_MAYSHARE)
return false;
if (is_vma_resv_set(vma, HPAGE_RESV_OWNER))
return false;
return false; <--------------------- [1]
}
Move on. If I read it right, above [1] is exactly for avoid_reserve==1
case, where it basically says "it's !NORESERVE, private, and it's not the
vma resv owner, either fork() or CoW". If my reading is correct, it means
after your patch 2, [1] should never be reachable at all.. I would hope
adding a panic() right above would be ok.
And irrelevant of whether my understanding is correct.. math-wise above is
also already the same as:
static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg,
bool avoid_reserve)
{
if (avoid_reserve)
return false;
if (chg == 0)
return true;
return false;
}
Then it makes a lot more sense now, because afaict, gbl_chg is exactly what
should_use_hstate_resv() is looking for.. only except avoid_reserve==true.
Would above make sense to you?
In short, it's about whether a patch on top of your series would further
simply this whole thing, like below:
===8<===
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 60e72214d5bf..4b1c5c4ed7be 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1245,80 +1245,6 @@ void clear_vma_resv_huge_pages(struct vm_area_struct *vma)
hugetlb_dup_vma_private(vma);
}
-/*
- * Returns true if this allocation should use (debit) hstate reservations, based on
- *
- * @vma: VMA config
- * @chg: Whether the page requirement can be satisfied using subpool reservations
- * @avoid_reserve: Whether allocation was requested to avoid using reservations
- */
-static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg,
- bool avoid_reserve)
-{
- if (avoid_reserve)
- return false;
-
- if (vma->vm_flags & VM_NORESERVE) {
- /*
- * This address is already reserved by other process(chg == 0),
- * so, we should decrement reserved count. Without decrementing,
- * reserve count remains after releasing inode, because this
- * allocated page will go into page cache and is regarded as
- * coming from reserved pool in releasing step. Currently, we
- * don't have any other solution to deal with this situation
- * properly, so add work-around here.
- */
- if (vma->vm_flags & VM_MAYSHARE && chg == 0)
- return true;
- else
- return false;
- }
-
- /* Shared mappings always use reserves */
- if (vma->vm_flags & VM_MAYSHARE) {
- /*
- * We know VM_NORESERVE is not set. Therefore, there SHOULD
- * be a region map for all pages. The only situation where
- * there is no region map is if a hole was punched via
- * fallocate. In this case, there really are no reserves to
- * use. This situation is indicated if chg != 0.
- */
- if (chg)
- return false;
- else
- return true;
- }
-
- /*
- * Only the process that called mmap() has reserves for private
- * mappings. A child process with MAP_PRIVATE mappings created by their
- * parent have no page reserves.
- */
- if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
- /*
- * Like the shared case above, a hole punch or truncate
- * could have been performed on the private mapping.
- * Examine the value of chg to determine if reserves
- * actually exist or were previously consumed.
- * Very Subtle - The value of chg comes from a previous
- * call to vma_needs_reserves(). The reserve map for
- * private mappings has different (opposite) semantics
- * than that of shared mappings. vma_needs_reserves()
- * has already taken this difference in semantics into
- * account. Therefore, the meaning of chg is the same
- * as in the shared case above. Code could easily be
- * combined, but keeping it separate draws attention to
- * subtle differences.
- */
- if (chg)
- return false;
- else
- return true;
- }
-
- return false;
-}
-
static void enqueue_hugetlb_folio(struct hstate *h, struct folio *folio)
{
int nid = folio_nid(folio);
@@ -3255,7 +3181,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
}
- use_hstate_resv = should_use_hstate_resv(vma, gbl_chg, avoid_reserve);
+ use_hstate_resv = avoid_reserve || !gbl_chg;
/*
* charge_cgroup_reservation if this allocation is not consuming a
===8<===
Thanks,
--
Peter Xu
