On Mon, Jul 29, 2019 at 2:10 PM William Kucharski <william.kucharski@xxxxxxxxxx> wrote: > > Add filemap_huge_fault() to attempt to satisfy page faults on > memory-mapped read-only text pages using THP when possible. > > Signed-off-by: William Kucharski <william.kucharski@xxxxxxxxxx> [..] > +/** > + * filemap_huge_fault - read in file data for page fault handling to THP > + * @vmf: struct vm_fault containing details of the fault > + * @pe_size: large page size to map, currently this must be PE_SIZE_PMD > + * > + * filemap_huge_fault() is invoked via the vma operations vector for a > + * mapped memory region to read in file data to a transparent huge page during > + * a page fault. > + * > + * If for any reason we can't allocate a THP, map it or add it to the page > + * cache, VM_FAULT_FALLBACK will be returned which will cause the fault > + * handler to try mapping the page using a PAGESIZE page, usually via > + * filemap_fault() if so speicifed in the vma operations vector. > + * > + * Returns either VM_FAULT_FALLBACK or the result of calling allcc_set_pte() > + * to map the new THP. > + * > + * NOTE: This routine depends upon the file system's readpage routine as > + * specified in the address space operations vector to recognize when it > + * is being passed a large page and to read the approprate amount of data > + * in full and without polluting the page cache for the large page itself > + * with PAGESIZE pages to perform a buffered read or to pollute what > + * would be the page cache space for any succeeding pages with PAGESIZE > + * pages due to readahead. > + * > + * It is VITAL that this routine not be enabled without such filesystem > + * support. Rather than a hopeful comment, this wants an explicit mechanism to prevent inadvertent mismatched ->readpage() assumptions. Either a new ->readhugepage() op, or a flags field in 'struct address_space_operations' indicating that the address_space opts into being careful to handle huge page arguments. I.e. something like mmap_supported_flags that was added to 'struct file_operations'.
