__do_fault (memory.c, 3.1.1)
as this function creates new mapping for a page which does not exist yet, TLB entries are not altered.
This function has two parts:
1. allocate a page and prepare it appropriately.
2. fix page tables to point to this page.
In the first if block we see there are three tasks:
1. prepare the anonymous vma
2. allocate the page
3. register the page with memory control group
if unable, release the page
in all three cases, return VM_FAULT_OOM on failure.
the COW page is allocated before any other processing because it will reduce the lock_page() holding time on page cache.
how to detect, if it is a COW request: FAULT_FLAG_WRITE and the vma is NOT shared. COW is specially used when there is a fork from parent and a new process is created. but the memory pages are not allocated right away. instead, the virtual memory regions are marked not sharable and the page table entry is marked read only. so next time there is a page fault, it can detect COW.
after fixing COW page, the function prepares vmf structure. (vm_fault)
next, the fs fault handler is invoked. when the control reaches __do_fault, it is already decided that there is fs code involved.
...
the backing address space might want to know that the page is about to become writable or not. the filesystem code implements this functionality. in that case, the vma->vm_ops->page_mkwrite function will be present.
as this function creates new mapping for a page which does not exist yet, TLB entries are not altered.
This function has two parts:
1. allocate a page and prepare it appropriately.
2. fix page tables to point to this page.
In the first if block we see there are three tasks:
1. prepare the anonymous vma
2. allocate the page
3. register the page with memory control group
if unable, release the page
in all three cases, return VM_FAULT_OOM on failure.
the COW page is allocated before any other processing because it will reduce the lock_page() holding time on page cache.
how to detect, if it is a COW request: FAULT_FLAG_WRITE and the vma is NOT shared. COW is specially used when there is a fork from parent and a new process is created. but the memory pages are not allocated right away. instead, the virtual memory regions are marked not sharable and the page table entry is marked read only. so next time there is a page fault, it can detect COW.
after fixing COW page, the function prepares vmf structure. (vm_fault)
next, the fs fault handler is invoked. when the control reaches __do_fault, it is already decided that there is fs code involved.
...
the backing address space might want to know that the page is about to become writable or not. the filesystem code implements this functionality. in that case, the vma->vm_ops->page_mkwrite function will be present.
