// SPDX-License-Identifier: MIT
/*
 * Copyright © 2024 Intel Corporation
 */

#include <drm/ttm/ttm_backup.h>

#include <linux/export.h>
#include <linux/page-flags.h>
#include <linux/swap.h>

/*
 * Need to map shmem indices to handle since a handle value
 * of 0 means error, following the swp_entry_t convention.
 */
static unsigned long ttm_backup_shmem_idx_to_handle(pgoff_t idx)
{
        return (unsigned long)idx + 1;
}

static pgoff_t ttm_backup_handle_to_shmem_idx(pgoff_t handle)
{
        return handle - 1;
}

/**
 * ttm_backup_drop() - release memory associated with a handle
 * @backup: The struct backup pointer used to obtain the handle
 * @handle: The handle obtained from the @backup_page function.
 */
void ttm_backup_drop(struct file *backup, pgoff_t handle)
{
        loff_t start = ttm_backup_handle_to_shmem_idx(handle);

        start <<= PAGE_SHIFT;
        shmem_truncate_range(file_inode(backup), start,
                             start + PAGE_SIZE - 1);
}

/**
 * ttm_backup_copy_page() - Copy the contents of a previously backed
 * up page
 * @backup: The struct backup pointer used to back up the page.
 * @dst: The struct page to copy into.
 * @handle: The handle returned when the page was backed up.
 * @intr: Try to perform waits interruptible or at least killable.
 *
 * Return: 0 on success, Negative error code on failure, notably
 * -EINTR if @intr was set to true and a signal is pending.
 */
int ttm_backup_copy_page(struct file *backup, struct page *dst,
                         pgoff_t handle, bool intr)
{
        struct address_space *mapping = backup->f_mapping;
        struct folio *from_folio;
        pgoff_t idx = ttm_backup_handle_to_shmem_idx(handle);

        from_folio = shmem_read_folio(mapping, idx);
        if (IS_ERR(from_folio))
                return PTR_ERR(from_folio);

        copy_highpage(dst, folio_file_page(from_folio, idx));
        folio_put(from_folio);

        return 0;
}

/**
 * ttm_backup_backup_page() - Backup a page
 * @backup: The struct backup pointer to use.
 * @page: The page to back up.
 * @writeback: Whether to perform immediate writeback of the page.
 * This may have performance implications.
 * @idx: A unique integer for each page and each struct backup.
 * This allows the backup implementation to avoid managing
 * its address space separately.
 * @page_gfp: The gfp value used when the page was allocated.
 * This is used for accounting purposes.
 * @alloc_gfp: The gfp to be used when allocating memory.
 *
 * Context: If called from reclaim context, the caller needs to
 * assert that the shrinker gfp has __GFP_FS set, to avoid
 * deadlocking on lock_page(). If @writeback is set to true and
 * called from reclaim context, the caller also needs to assert
 * that the shrinker gfp has __GFP_IO set, since without it,
 * we're not allowed to start backup IO.
 *
 * Return: A handle on success. Negative error code on failure.
 *
 * Note: This function could be extended to back up a folio and
 * implementations would then split the folio internally if needed.
 * Drawback is that the caller would then have to keep track of
 * the folio size- and usage.
 */
s64
ttm_backup_backup_page(struct file *backup, struct page *page,
                       bool writeback, pgoff_t idx, gfp_t page_gfp,
                       gfp_t alloc_gfp)
{
        struct address_space *mapping = backup->f_mapping;
        unsigned long handle = 0;
        struct folio *to_folio;
        int ret;

        to_folio = shmem_read_folio_gfp(mapping, idx, alloc_gfp);
        if (IS_ERR(to_folio))
                return PTR_ERR(to_folio);

        folio_mark_accessed(to_folio);
        folio_lock(to_folio);
        folio_mark_dirty(to_folio);
        copy_highpage(folio_file_page(to_folio, idx), page);
        handle = ttm_backup_shmem_idx_to_handle(idx);

        if (writeback && !folio_mapped(to_folio) &&
            folio_clear_dirty_for_io(to_folio)) {
                folio_set_reclaim(to_folio);
                ret = shmem_writeout(to_folio, NULL, NULL);
                if (!folio_test_writeback(to_folio))
                        folio_clear_reclaim(to_folio);
                /*
                 * If writeout succeeds, it unlocks the folio.  errors
                 * are otherwise dropped, since writeout is only best
                 * effort here.
                 */
                if (ret)
                        folio_unlock(to_folio);
        } else {
                folio_unlock(to_folio);
        }

        folio_put(to_folio);

        return handle;
}

/**
 * ttm_backup_fini() - Free the struct backup resources after last use.
 * @backup: Pointer to the struct backup whose resources to free.
 *
 * After a call to this function, it's illegal to use the @backup pointer.
 */
void ttm_backup_fini(struct file *backup)
{
        fput(backup);
}

/**
 * ttm_backup_bytes_avail() - Report the approximate number of bytes of backup space
 * left for backup.
 *
 * This function is intended also for driver use to indicate whether a
 * backup attempt is meaningful.
 *
 * Return: An approximate size of backup space available.
 */
u64 ttm_backup_bytes_avail(void)
{
        /*
         * The idea behind backing up to shmem is that shmem objects may
         * eventually be swapped out. So no point swapping out if there
         * is no or low swap-space available. But the accuracy of this
         * number also depends on shmem actually swapping out backed-up
         * shmem objects without too much buffering.
         */
        return (u64)get_nr_swap_pages() << PAGE_SHIFT;
}
EXPORT_SYMBOL_GPL(ttm_backup_bytes_avail);

/**
 * ttm_backup_shmem_create() - Create a shmem-based struct backup.
 * @size: The maximum size (in bytes) to back up.
 *
 * Create a backup utilizing shmem objects.
 *
 * Return: A pointer to a struct file on success,
 * an error pointer on error.
 */
struct file *ttm_backup_shmem_create(loff_t size)
{
        return shmem_file_setup("ttm shmem backup", size,
                                EMPTY_VMA_FLAGS);
}