On 09/02/2018 22:44, Benjamin Herrenschmidt wrote:
>>
>> (Also, why is this code reinventing most of hva_to_pfn? Not asking you
>> to fix that of course, but perhaps there's something to improve in the
>> generic code too).
> I've been wondering the same thing, which led me to try to understand
> hva_to_pfn, which resulted in skull marks in the wall ;-)
>
> So hva_to_pfn does a complicated dance around 3 or 4 different variants
> of gup and it's really not obvious why, the "intent" doesn't appear to
> be documented clearly. I think I somewhat figued out it relates to the
> async page fault stuff but even then I don't actually know what those
> are and what is their purpose :-)
Yeah, hva_to_pfn has all those arguments to tweak its behavior, however
you can treat it as essentially get_user_pages_unlocked:
- if async==false && atomic==false you can ignore hva_to_pfn_fast.
(hva_to_pfn_fast uses __get_user_pages_fast)
- hva_to_pfn_slow then is essentially get_user_pages_unlocked.
Optionally it's followed by __get_user_pages_fast to get a writable
mapping if the caller would like it but does not require it, but you can
ignore this if you pass writable==NULL.
PPC uses get_user_pages_fast; x86 cannot use it directly because we need
FOLL_HWPOISON and get_user_pages_fast does not support it.
However, get_user_pages_fast is itself a combination of
__get_user_pages_fast and get_user_pages_unlocked. So if it is useful
for PPC to use get_user_pages_fast, perhaps the generic code should go
down hva_to_pfn_fast unconditionally, even if async and atomic are both
false.
The other big difference is that you follow up with the PageHuge check
to get compound_head/compound_order. This is a bit like
kvm_host_page_size, but different. It would be fine for me to add a
struct page ** argument to hva_to_pfn if that's enough to remove the
duplicate code in arch/powerpc/kvm/.
See the untested/uncompiled patch below the signature for some ideas.
Thanks,
Paolo
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b4414842b023..a2c8824ae608 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1339,15 +1339,12 @@ static inline int check_user_page_hwpoison(unsigned long addr)
* The atomic path to get the writable pfn which will be stored in @pfn,
* true indicates success, otherwise false is returned.
*/
-static bool hva_to_pfn_fast(unsigned long addr, bool atomic, bool *async,
- bool write_fault, bool *writable, kvm_pfn_t *pfn)
+static struct page *__hva_to_page_fast(unsigned long addr, bool write_fault,
+ bool *writable)
{
struct page *page[1];
int npages;
- if (!(async || atomic))
- return false;
-
/*
* Fast pin a writable pfn only if it is a write fault request
* or the caller allows to map a writable pfn for a read fault
@@ -1357,23 +1354,21 @@ static bool hva_to_pfn_fast(unsigned long addr, bool atomic, bool *async,
return false;
npages = __get_user_pages_fast(addr, 1, 1, page);
- if (npages == 1) {
- *pfn = page_to_pfn(page[0]);
+ if (npages < 0)
+ return ERR_PTR(npages);
- if (writable)
- *writable = true;
- return true;
- }
+ if (writable)
+ *writable = true;
- return false;
+ return page[0];
}
/*
* The slow path to get the pfn of the specified host virtual address,
* 1 indicates success, -errno is returned if error is detected.
*/
-static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
- bool *writable, kvm_pfn_t *pfn)
+static struct page *hva_to_page_unlocked(unsigned long addr, bool *async,
+ bool write_fault, bool *writable)
{
struct page *page[1];
int npages = 0;
@@ -1395,24 +1390,20 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
npages = get_user_pages_unlocked(addr, 1, page, flags);
}
- if (npages != 1)
- return npages;
+ if (npages < 0)
+ return ERR_PTR(npages);
/* map read fault as writable if possible */
if (unlikely(!write_fault) && writable) {
- struct page *wpage[1];
-
- npages = __get_user_pages_fast(addr, 1, 1, wpage);
- if (npages == 1) {
- *writable = true;
+ struct page *wpage;
+ wpage = __hva_to_page_fast(addr, write_fault, writable);
+ if (!IS_ERR(wpage)) {
put_page(page[0]);
- page[0] = wpage[0];
+ return wpage;
}
-
- npages = 1;
}
- *pfn = page_to_pfn(page[0]);
- return npages;
+
+ return page[0];
}
static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
@@ -1487,33 +1478,37 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
* whether the mapping is writable.
*/
static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
- bool write_fault, bool *writable)
+ bool write_fault, bool *writable, struct page **p_page)
{
struct vm_area_struct *vma;
kvm_pfn_t pfn = 0;
- int npages, r;
+ struct page *page;
+ int r;
/* we can do it either atomically or asynchronously, not both */
BUG_ON(atomic && async);
- if (hva_to_pfn_fast(addr, atomic, async, write_fault, writable, &pfn))
- return pfn;
+ page = __hva_to_page_fast(addr, write_fault, writable);
+ if (!IS_ERR(page))
+ goto got_page;
if (atomic)
return KVM_PFN_ERR_FAULT;
- npages = hva_to_pfn_slow(addr, async, write_fault, writable, &pfn);
- if (npages == 1)
- return pfn;
+ page = hva_to_page_unlocked(addr, async, write_fault, writable);
+ if (!IS_ERR(page))
+ goto got_page;
down_read(¤t->mm->mmap_sem);
- if (npages == -EHWPOISON ||
+ /* FIXME, is check_user_page_hwpoison still needed? */
+ if (PTR_ERR(page) == -EHWPOISON ||
(!async && check_user_page_hwpoison(addr))) {
- pfn = KVM_PFN_ERR_HWPOISON;
- goto exit;
+ up_read(¤t->mm->mmap_sem);
+ return KVM_PFN_ERR_HWPOISON;
}
retry:
+ *p_page = NULL;
vma = find_vma_intersection(current->mm, addr, addr + 1);
if (vma == NULL)
@@ -1529,9 +1523,13 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
*async = true;
pfn = KVM_PFN_ERR_FAULT;
}
-exit:
up_read(¤t->mm->mmap_sem);
return pfn;
+
+got_page:
+ if (p_page)
+ *p_page = page;
+ return page_to_pfn(page);
}
kvm_pfn_t __gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn,
@@ -1559,7 +1557,7 @@ kvm_pfn_t __gfn_to_pfn_memslot(struct kvm_memory_slot *slot, gfn_t gfn,
}
return hva_to_pfn(addr, atomic, async, write_fault,
- writable);
+ writable, NULL);
}
EXPORT_SYMBOL_GPL(__gfn_to_pfn_memslot);
--
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